WarmTransfer
sbc trunking · published

Calls-per-second limits on SIP trunks and dialers

Verified 2026-10-02 · 50 sources · tier 1–3 · 1 disputed

Also known as CPS limit, calls per second.

Disputed: 1 source below carries conflicting evidence.

Twilio Elastic SIP Trunking provides 1 termination (outbound) call per second per trunk per region by default 49, while Telnyx limits standard SIP trunking traffic to 20 CPS per source IP address or SIP username by default 31. Both reject outbound traffic over the limit with SIP 503 3829.

How CPS limits are defined

Telnyx defines CPS as the number of outbound call attempts initiated within a single second 30. Twilio error 32001 means the calls per second placed against a SIP trunk exceeded its set limit 35. Genesys Cloud trunk Common settings include Max Call Rate, the average number of calls permitted per second on the trunk 17.

Kamailio's ratelimit module computes rates over timer_interval, which defaults to 10 seconds 22. Kamailio's pike module flags a source IP as flooding when it exceeds reqs_density_per_unit (default 30) requests per sampling_time_unit (default 2 seconds) 20.

Twilio Elastic SIP Trunking states unlimited concurrent calls, except that new accounts without an approved Business Profile have limited concurrent calls 40. WarmTransfer's reading of the sources is that on Twilio Elastic SIP Trunking, CPS and concurrent-call limits are enforced separately, so a trunk with spare concurrent capacity can still reject calls on rate alone 12.

See also SIP trunk capacity and concurrent call planning.

Carrier trunk limits

Platform Documented default
Twilio Elastic SIP Trunking 1 termination CPS per trunk per region 49
Telnyx SIP trunking 20 CPS per source IP address or SIP username 31
Bandwidth Programmable Voice API 5 calls per second and 100 active sessions per account 10
Genesys Cloud outbound dialing 15 calls dialed per second per organization 19

Twilio

In the Twilio Console a trunk's termination CPS can be self-served to a value between 1 and 5, and higher values require contacting Twilio sales 50. Sources disagree on that ceiling: Twilio's August 2020 changelog announced that termination CPS could be self-served immediately up to 15 CPS per trunk per region, while current Twilio documentation states a self-serve ceiling of 5 39. Twilio's scale-and-limits page notes that termination CPS increases are dependent on carrier support 42.

To qualify for increased Twilio termination CPS, the traffic profile must meet these guidelines: 454746

  • Average call duration must be greater than 30 seconds 45.
  • No more than 10% of calls may last 12 seconds or less 47.
  • Answer-seizure ratio (ASR) must be greater than 70% 46.

Twilio charges a monthly fee for termination CPS above the default, prorated when the increase is made mid-month 48. For inbound traffic, Twilio Elastic SIP Trunking states unlimited origination calls per second 44.

See also Setting up an Asterisk PBX with Twilio Elastic SIP Trunking.

Telnyx

Telnyx bills an outbound peak CPS surcharge based on the 95th percentile of hourly peak CPS values, excluding hours with zero activity 34. The surcharge is graduated: the first 5 CPS are free, then $12 per CPS for 6-25, $16 for 26-200, $24 for 201-250 and $30 above 250 33. Telnyx excludes rejected (rate-limited) attempts, internal retries and Programmable Voice traffic from the CPS surcharge calculation 32.

See also SIP trunk pricing models.

Bandwidth

Bandwidth SIP trunking call concurrency limits are set by opening a support ticket and are calculated from traffic type and expected minutes of use 8. The Bandwidth Programmable Voice API sets a default rate limit of 5 calls per second and 100 active sessions per account 10. Bandwidth Programmable Voice can queue outbound calls for up to 5 minutes, with queue capacity equal to the CPS limit multiplied by 300 seconds 11.

Dialer and contact-center limits

Genesys Cloud

Genesys Cloud outbound dialing is limited to 15 calls dialed per second per organization by default 19. Since March 2025 each Genesys Cloud outbound campaign is capped at 50 CPS in all regions, and the cap matters only when the organization CPS exceeds 50 14. To raise outbound CPS, a customer opens a care case giving the CPS wanted, telephony model and business justification, and Genesys aims to turn requests around in 10 business days 15.

On Genesys Cloud BYOC Cloud trunks, Max Concurrent Calls defaults to Unlimited and Genesys recommends leaving it Unlimited 13.

Amazon Connect

Amazon Connect's Concurrent active calls per instance quota defaults to 10 for new accounts, includes PSTN and WebRTC calls, and is adjustable 5. The Concurrent campaign active calls per instance quota defaults to 0 and must be raised before outbound campaigns can place calls 4.

Amazon Connect Service API operations not listed as exceptions, which includes StartOutboundVoiceContact, have a RateLimit of 2 requests per second and a BurstLimit of 5, shared per account per Region 2. The outbound campaigns operations PutDialRequestBatch and PutOutboundRequestBatch have a RateLimit and BurstLimit of 10 requests per second per account per Region 3.

Diagnosing a rate-limit rejection

  • Twilio rejects termination calls sent faster than the trunk's CPS with a SIP 503 response, documented as '503 Trunk CPS limit exceeded' 38.
  • Twilio logs debugger error 32001 for the trunk-level limit 41 and debugger error 32012 when the parent account's pooled termination CPS limit is exceeded 36.
  • Twilio returns '503 Trunk concurrent call limit exceeded' when a trial account or one without an approved Business Profile exceeds 4 simultaneous termination calls 37.
  • Telnyx rejects traffic over the real-time CPS limit with SIP 503 and a documented reason phrase 29.
  • When a Bandwidth call concurrency limit is reached, the call receives either SIP 503 Service Unavailable or 486 Busy Here 7.
  • Bandwidth Programmable Voice returns HTTP 429 with 'Account call creation rate limit exceeded' or 'Account concurrent call limit exceeded' when outbound call creation exceeds limits 9.
  • By default a Genesys Cloud trunk returns 503 Service Unavailable when its Max Call threshold is exceeded 16, and administrators can replace it with another 4xx, 5xx or 6xx code via Max Call Reason Code, for example 486 Busy Here 18.
  • When Amazon Connect's Concurrent active calls per instance quota is exceeded, contacts get a reorder (fast busy) tone 6, and exceeding the Concurrent campaign active calls per instance quota gives callers a fast busy tone 4.

WarmTransfer's reading of the sources is that a bare SIP 503 from Twilio Elastic SIP Trunking does not distinguish a CPS breach from a concurrency breach or an outage, so the reason phrase or carrier-side logs are needed 1.

RFC 3261 says a client receiving 503 Service Unavailable should try an alternate server and should not send further requests to that server for the Retry-After period if one is given 24. RFC 3261 defines 486 Busy Here as the callee's end system being reached but unwilling or unable to take additional calls, a callee condition rather than a server-capacity one 23.

See also SIP 503 and trunk out-of-service failures.

Protocol-level overload control

RFC 7339 states that the 503 mechanism cannot prevent overload of a SIP server or congestion collapse, and can make traffic oscillate between servers 26. RFC 7339 defines the Via header parameters oc, oc-algo, oc-validity and oc-seq so a server can signal overload to its upstream client, and makes loss-based control mandatory to implement 27. RFC 7415 adds rate-based SIP overload control in which the oc value is a maximum rate in messages per second, selected by the oc-algo token 'rate' 28.

On a self-hosted proxy, Kamailio's ratelimit module limits SIP requests per request type using TAILDROP, RED, NETWORK or FEEDBACK algorithms, and its example config answers excess requests with 503 'Limiting' 21.

Applicability

Applies to: Twilio Elastic SIP Trunking, Telnyx Elastic SIP Trunking, Genesys Cloud, Kamailio, Bandwidth Programmable Voice, Bandwidth SIP Trunking, and AWS Amazon Connect. Deployments: on-premises and multi-tenant. Sources checked 2026-10-02. Twilio's default termination CPS applies per trunk per region 49. The Genesys Cloud per-campaign 50 CPS cap applies since March 2025 in all regions 14. Amazon Connect API throttling quotas are shared per account per Region 23.

What remains uncertain

Sources disagree on Twilio's self-serve termination CPS ceiling, so verify the current Console limit before planning around it 39. A calls-per-second quota specific to Amazon Connect outbound campaigns is not covered by the sources below. A method for sizing CPS against concurrent-call capacity is not covered by the sources below. A default CPS limit for Bandwidth SIP trunking, as distinct from its Programmable Voice API, is not covered by the sources below. Pacing and CPS behaviour of outbound dialer platforms other than those named in the claims is not covered by the sources below. CPS limits on trunks in cloud calling platforms other than those named in the claims are not covered by the sources below. CPS-based call admission control on session border controllers is not covered by the sources below. A Telnyx outbound channel or concurrent-call limit is not covered by the sources below.

See also

Related to

Referenced by

  • SIP trunk pricing models — Some metered trunk offers sell calls-per-second headroom as a separately priced monthly add-on (Twilio termination CPS).

Sources

  1. 1
    A bare SIP 503 from Twilio Elastic SIP Trunking does not distinguish a CPS breach from a concurrency breach or an outage; the reason phrase or carrier-side logs are needed.inferred
    Troubleshooting your Trunk · Termination calls: both 503 headings · Checked 2026-10-02
  2. 2
    Amazon Connect Service API operations not listed as exceptions, which includes StartOutboundVoiceContact, have a RateLimit of 2 requests per second and BurstLimit of 5, shared per account per Region.
    Connect Customer service quotas · API throttling quotas: Connect Customer API throttling quotas · Checked 2026-10-02
  3. 3
    Amazon Connect outbound campaigns PutDialRequestBatch and PutOutboundRequestBatch have a RateLimit and BurstLimit of 10 requests per second per account per Region.
    Connect Customer service quotas · Outbound Campaigns Service API throttling quotas · Checked 2026-10-02
  4. 4
    Amazon Connect's Concurrent campaign active calls per instance quota defaults to 0 and must be raised before outbound campaigns can place calls; exceeding it gives callers a fast busy tone.
    Connect Customer service quotas · Outbound campaigns service quotas table · Checked 2026-10-02
  5. 5
    Amazon Connect's Concurrent active calls per instance quota defaults to 10 for new accounts, includes PSTN and WebRTC calls, and is adjustable.
    Connect Customer service quotas · Connect Customer quotas table: Concurrent active calls per instance · Checked 2026-10-02
  6. 6
    When Amazon Connect's Concurrent active calls per instance quota is exceeded, contacts get a reorder (fast busy) tone.
    Connect Customer service quotas · How contacts are counted · Checked 2026-10-02
  7. 7
    When a Bandwidth call concurrency limit is reached, the call receives either SIP 503 Service Unavailable or 486 Busy Here.
    Outbound calling integration guide · Call concurrency section · Checked 2026-10-02
  8. 8
    Bandwidth SIP trunking call concurrency limits are set by opening a support ticket and are calculated from traffic type and expected minutes of use.
    Outbound calling integration guide · Call concurrency section · Checked 2026-10-02
  9. 9
    Bandwidth Programmable Voice returns HTTP 429 with 'Account call creation rate limit exceeded' or 'Account concurrent call limit exceeded' when outbound call creation exceeds limits.
    Rate Limits · 429 response section · Checked 2026-10-02
  10. 10
    Bandwidth Programmable Voice API sets a default rate limit of 5 calls per second and 100 active sessions per account.
    Rate Limits · Rate Limits page, default limits · Checked 2026-10-02
  11. 11
    Bandwidth Programmable Voice can queue outbound calls for up to 5 minutes, with queue capacity equal to the CPS limit multiplied by 300 seconds.
    Rate Limits · outbound call queuing section · Checked 2026-10-02
  12. 12
    On Twilio Elastic SIP Trunking, CPS and concurrent-call limits are enforced separately, so a trunk with spare concurrent capacity can still reject calls on rate alone.inferred
    Troubleshooting your Trunk · Termination calls: separate CPS and concurrent-call 503 headings · Checked 2026-10-02
  13. 13
    On Genesys Cloud BYOC Cloud trunks Max Concurrent Calls defaults to Unlimited and Genesys recommends leaving it Unlimited.
    Configure maximum call settings · BYOC Cloud: Max Concurrent Calls · Checked 2026-10-02
  14. 14
    Since March 2025 each Genesys Cloud outbound campaign is capped at 50 CPS in all regions; the cap matters only when the organization CPS exceeds 50.
    Outbound campaign limits · per-campaign CPS limit entry · Checked 2026-10-02
  15. 15
    To raise Genesys Cloud outbound CPS, a customer opens a care case giving the CPS wanted, telephony model and business justification; Genesys aims to turn requests around in 10 business days.
    Outbound campaign limits · requesting higher CPS · Checked 2026-10-02
  16. 16
    By default a Genesys Cloud trunk returns 503 Service Unavailable when its Max Call threshold is exceeded.
    Configure maximum call settings · Max Call Reason Code description · Checked 2026-10-02
  17. 17
    Genesys Cloud trunk Common settings include Max Call Rate, the average number of calls permitted per second on the trunk.
    Configure maximum call settings · Common settings: Max Call Rate · Checked 2026-10-02
  18. 18
    Genesys Cloud lets administrators replace the default 503 with another 4xx, 5xx or 6xx code via Max Call Reason Code, for example 486 Busy Here.
    Configure maximum call settings · Max Call Reason Code description · Checked 2026-10-02
  19. 19
    Genesys Cloud outbound dialing is limited to 15 calls dialed per second per organization by default.
    Outbound campaign limits · organization CPS limit entry · Checked 2026-10-02
  20. 20
    Kamailio's pike module flags a source IP as flooding when it exceeds reqs_density_per_unit (default 30) requests per sampling_time_unit (default 2 seconds).
    Pike Module · Sections 3.1 and 3.2 · Checked 2026-10-02
  21. 21
    Kamailio's ratelimit module limits SIP requests per request type using TAILDROP, RED, NETWORK or FEEDBACK algorithms, and its example config answers excess requests with 503 'Limiting'.
    Ratelimit Module · overview, algorithms section, and functions example · Checked 2026-10-02
  22. 22
    Kamailio ratelimit computes rates over timer_interval, which defaults to 10 seconds.
    Ratelimit Module · Parameters section: timer_interval · Checked 2026-10-02
  23. 23
    RFC 3261 defines 486 Busy Here as the callee's end system being reached but unwilling or unable to take additional calls, a callee condition rather than a server-capacity one.
    RFC 3261 — SIP: Session Initiation Protocol · Section 21.4.24 · Checked 2026-10-02
  24. 24
    RFC 3261 says a client receiving 503 Service Unavailable should try an alternate server and should not send further requests to that server for the Retry-After period if one is given.
    RFC 3261 — SIP: Session Initiation Protocol · Section 21.5.4 · Checked 2026-10-02
  25. 25
    RFC 3261 permits Retry-After with 500, 503 or 600 responses to tell the caller how long to wait before retrying.
    RFC 3261 — SIP: Session Initiation Protocol · Section 20.33 · Checked 2026-10-02
  26. 26
    RFC 7339 states that the 503 mechanism cannot prevent overload of a SIP server or congestion collapse, and can make traffic oscillate between servers.
    RFC 7339: Session Initiation Protocol (SIP) Overload Control · Section 1 · Checked 2026-10-02
  27. 27
    RFC 7339 defines Via header parameters oc, oc-algo, oc-validity and oc-seq so a server can signal overload to its upstream client, and makes loss-based control mandatory to implement.
    RFC 7339: Session Initiation Protocol (SIP) Overload Control · Sections 4.1-4.4 and 7 · Checked 2026-10-02
  28. 28
    RFC 7415 adds rate-based SIP overload control in which the oc value is a maximum rate in messages per second, selected by oc-algo token 'rate'.
    RFC 7415: Session Initiation Protocol (SIP) Rate Control · Sections 3.2 and 3.3 · Checked 2026-10-02
  29. 29
    Telnyx rejects traffic over the real-time CPS limit with SIP 503 and reason phrase 'CPS Limit Reached (P05)'.
    Calls per second (CPS) surcharge · real-time outbound CPS limits section · Checked 2026-10-02
  30. 30
    Telnyx defines CPS as the number of outbound call attempts initiated within a single second.
    Calls per second (CPS) surcharge · definition section · Checked 2026-10-02
  31. 31
    Telnyx limits standard SIP trunking traffic to 20 CPS per source IP address or SIP username by default.
    Calls per second (CPS) surcharge · real-time outbound CPS limits section · Checked 2026-10-02
  32. 32
    Telnyx excludes rejected (rate-limited) attempts, internal retries and Programmable Voice traffic from the CPS surcharge calculation.
    Calls per second (CPS) surcharge · excluded traffic section · Checked 2026-10-02
  33. 33
    Telnyx's CPS surcharge is graduated: first 5 CPS free, then $12 per CPS for 6-25, $16 for 26-200, $24 for 201-250 and $30 above 250.
    Calls per second (CPS) surcharge · pricing bands table · Checked 2026-10-02
  34. 34
    Telnyx bills an outbound peak CPS surcharge based on the 95th percentile of hourly peak CPS values, excluding hours with zero activity.
    Calls per second (CPS) surcharge · monthly surcharge method and measurement steps · Checked 2026-10-02
  35. 35
    Twilio error 32001 means the calls per second placed against a SIP trunk exceeded its set limit.
    32001: SIP: Trunk CPS limit exceeded · error description · Checked 2026-10-02
  36. 36
    Twilio logs debugger error 32012 when the parent account's pooled termination CPS limit is exceeded.
    Calls per Second (CPS) — Trunking Termination · section on exceeding CPS / debugger alerts · Checked 2026-10-02
  37. 37
    Twilio returns '503 Trunk concurrent call limit exceeded' when a trial account or one without an approved Business Profile exceeds 4 simultaneous termination calls.
    Troubleshooting your Trunk · Termination calls: 'The call fails with a 503 Trunk concurrent call limit exceeded error' · Checked 2026-10-02
  38. 38
    Twilio rejects termination calls sent faster than the trunk's CPS with a SIP 503 response, documented as '503 Trunk CPS limit exceeded'.
    Troubleshooting your Trunk · Termination calls: 'The call fails with a 503 Trunk CPS limit exceeded error' · Checked 2026-10-02
  39. 39
    Twilio's August 2020 changelog announced that termination CPS could be self-served immediately up to 15 CPS per trunk per region; current Twilio documentation states a self-serve ceiling of 5.disputed
    Upgrade your Elastic SIP Trunking calls-per-second (CPS) on-demand · changelog entry body · Checked 2026-10-02
  40. 40
    Twilio Elastic SIP Trunking states unlimited concurrent calls, except that new accounts without an approved Business Profile have limited concurrent calls.
    Twilio SIP Trunking Scale and Limits · Accounts section · Checked 2026-10-02
  41. 41
    When a Twilio trunk's termination CPS is exceeded, calls start to be rejected and Twilio logs debugger error 32001 for the trunk-level limit.
    Calls per Second (CPS) — Trunking Termination · section on exceeding CPS / debugger alerts · Checked 2026-10-02
  42. 42
    Twilio's scale-and-limits page notes that termination CPS increases are dependent on carrier support.
    Twilio SIP Trunking Scale and Limits · Accounts section, termination CPS bullet · Checked 2026-10-02
  43. 43
    For Twilio trunks in regions other than Ashburn (US1), increasing CPS requires parent-account pooled CPS.
    Calls per Second (CPS) — Trunking Termination · regions section · Checked 2026-10-02
  44. 44
    Twilio Elastic SIP Trunking states unlimited origination (inbound) calls per second.
    Twilio SIP Trunking Scale and Limits · Accounts section · Checked 2026-10-02
  45. 45
    To qualify for increased Twilio termination CPS, average call duration must be greater than 30 seconds.
    Calls per Second (CPS) — Trunking Termination · traffic profile guidelines list · Checked 2026-10-02
  46. 46
    To qualify for increased Twilio termination CPS, answer-seizure ratio (ASR) must be greater than 70%.
    Calls per Second (CPS) — Trunking Termination · traffic profile guidelines list · Checked 2026-10-02
  47. 47
    To qualify for increased Twilio termination CPS, no more than 10% of calls may last 12 seconds or less.
    Calls per Second (CPS) — Trunking Termination · traffic profile guidelines list · Checked 2026-10-02
  48. 48
    Twilio charges a monthly fee for termination CPS above the default, prorated when the increase is made mid-month.
    Calls per Second (CPS) — Trunking Termination · pricing section · Checked 2026-10-02
  49. 49
    Twilio Elastic SIP Trunking provides 1 termination (outbound) call per second per trunk per region by default.
    Calls per Second (CPS) — Trunking Termination · opening section (default CPS) · Checked 2026-10-02
  50. 50
    In the Twilio Console a trunk's termination CPS can be self-served to a value between 1 and 5; higher values require contacting Twilio sales.
    Calls per Second (CPS) — Trunking Termination · section on increasing CPS via Console · Checked 2026-10-02

Documents

tier 1 standards and regulators

RFC 3261 — SIP: Session Initiation Protocol

IETF / RFC Editor · 2002-06 · accessed 2026-09-04

tier 1 standards and regulators

RFC 7339: Session Initiation Protocol (SIP) Overload Control

IETF / RFC Editor · 2014-09-01 · accessed 2026-09-25

tier 1 standards and regulators

RFC 7415: Session Initiation Protocol (SIP) Rate Control

IETF · 2015-02-01 · accessed 2026-10-02

tier 2 current vendor documentation

32001: SIP: Trunk CPS limit exceeded

Twilio · accessed 2026-09-24

tier 2 current vendor documentation

Calls per Second (CPS) — Trunking Termination

Twilio · 2026-07-21 · accessed 2026-09-24

tier 2 current vendor documentation

Calls per second (CPS) surcharge

Telnyx · accessed 2026-10-02

tier 2 current vendor documentation

Configure maximum call settings

Genesys · accessed 2026-10-02

tier 2 current vendor documentation

Connect Customer service quotas

Amazon Web Services · accessed 2026-09-07

tier 2 current vendor documentation

Outbound calling integration guide

Bandwidth · 2025-11-17 · accessed 2026-10-02

tier 2 current vendor documentation

Outbound campaign limits

Genesys · accessed 2026-10-02

tier 2 current vendor documentation

Rate Limits

Bandwidth · accessed 2026-10-02

tier 2 current vendor documentation

Troubleshooting your Trunk

Twilio · 2026-05-06 · accessed 2026-09-24

tier 2 current vendor documentation

Twilio SIP Trunking Scale and Limits

Twilio · 2026-03-09 · accessed 2026-09-24

tier 2 current vendor documentation

Upgrade your Elastic SIP Trunking calls-per-second (CPS) on-demand

Twilio · 2020-08-31 · accessed 2026-10-02

tier 3 vendor-maintained repositories

Pike Module

Kamailio · accessed 2026-10-02

tier 3 vendor-maintained repositories

Ratelimit Module

Kamailio · accessed 2026-10-02

Cite this page

APA

WarmTransfer. (2026, October 2). Calls-per-second limits on SIP trunks and dialers. WarmTransfer. https://warmtransfer.net/knowledge/sip-trunk-cps-limits

BibTeX

@misc{warmtransfer-sip-trunk-cps-limits,
  title  = {Calls-per-second limits on SIP trunks and dialers},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sip-trunk-cps-limits},
  note   = {Verified 2026-10-02}
}