cisco onprem uc · published

Troubleshooting Unified CM media resource allocation failures

Verified 2026-09-30 · 58 sources · tier 2 · 1 disputed

Disputed: 1 source below carries conflicting evidence.

Unified CM makes media resources available to calls by placing them in a media resource group list assigned to a device pool or to an individual device 7. When a needed resource cannot be allocated, the documented outcomes include a call that connects without supplementary services, a failed call, or fast-busy tone after the user answers 40 8 53.

How allocation works

By default, an individual device uses the media resources assigned to its device pool 7. When a device has a media resource group list on the device itself and another on its device pool, Unified CM searches the device-level list first and then the device pool list 36.

A media resource group list is an ordered list of groups 37. Unified CM must exhaust all resources in one group before it tries a resource from the next group in the same list 37. All resources in one media resource group are treated as equal, and Unified CM load shares between resources of each type within the group 34.

A media resource that is not assigned to any media resource group is automatically placed in the default group 5. The default media resource list is always searched as the last resort, either when no resource is available in the device list and the device pool list, or when no list is configured at any level 6.

Resource types and stated capacities

Media termination points

When Unified CM determines that a call endpoint requires an MTP, it allocates the resource from the MTP that has the least active streams 39. Release 15 lists these reasons for Unified CM to insert an MTP in the media path 38:

To act as a Trusted Relay Point 38.
To convert between IPv4 and IPv6 for RTP streams 38.
To deliver SIP Early Offer over SIP trunks 38.
To address DTMF transport mismatches 38.
To act as an RSVP agent 38.

The Cisco IP Voice Media Streaming service must be running for the software MTP to be active 52. The Release 15 guide states that the Cisco Media Termination Point Software MTP type provides a default of 48 user-configurable MTP resources, depending on network speed and the network interface card 51. Sources disagree on software MTP capacity: the Release 12.5(1) troubleshooting guide states that one MTP software application supports 24 calls, which differs from the Release 15 figure 50.

Transcoders and Trusted Relay Points

During a call in which the endpoints use different voice codecs, Unified CM invokes a transcoder into the media path 57. In addition to codec conversion, a transcoder can provide the same functionality as a media termination point 58. A Trusted Relay Point is an MTP or transcoder that Unified CM can insert into the media stream as a control point, and it is enabled by the Trusted Relay Point check box on the MTP or transcoder configuration window 54.

Annunciator and conference bridges

The annunciator is activated on a cluster node by turning on the Cisco IP Voice Media Streaming service on that node 3. The software conference bridge is created automatically when a Unified CM server is created and cannot be deleted 48. Software conference devices support G.711 codecs by default 49. A Cisco IOS Enhanced conference bridge supports G.711 a-law/mu-law, G.729, G.729a, G.729b, G.729ab, GSM FR and GSM EFR participants 13.

Resource	Stated capacity
Annunciator	48 simultaneous media streams by default 2
Software conference bridge	At most 256 callers, illustrated in the Release 15 guide as 64 sessions of 4 parties 47
Cisco IOS Enhanced conference bridge	Up to 8 parties in a single call 13
Cisco IP Voice Media Streaming software (1 installation)	16 Unicast Available Conferences of 3 people each, per the Release 12.5(1) troubleshooting guide 17

What happens when allocation fails

If an MTP resource is not available when it is needed, the call connects without an MTP and that call has no supplementary services 40. In Release 15, whether Unified CM fails the call depends on Fail Call If Trusted Relay Point Allocation Fails and Fail Call If MTP Allocation Fails 8:

When Fail Call If Trusted Relay Point Allocation Fails is True, the call is failed regardless of the MTP parameter 8.
When it is False and Fail Call If MTP Allocation Fails is True, the call is failed if the MTP is required for an H.323 endpoint, but not if it is required for a SIP endpoint 8.
When both are False, the call is not failed 8.

A Trusted Relay Point is in most instances allocated after the call is answered, so a call that fails because the TRP cannot be allocated may give the user fast-busy tone after answering 53. The Release 12.5(1) troubleshooting guide gives the symptom text 'No Conference Bridge Available' and says it could indicate either a software or a hardware problem 42.

Codec selection and transcoding

To select an audio codec for a call, Unified CM takes the codecs matching on both sides of the call leg, filters out those exceeding the configured maximum audio bit rate for the region pair, and picks the preferred codec from what remains 45. The Release 12.5(1) troubleshooting guide attributes a reorder tone on going off hook to possible codec disagreement between regions 46. It directs verifying that both call ends support at least one common codec and using transcoders if they do not 46.

Music On Hold

The held party determines the media resource group list that Unified CM uses to allocate a Music On Hold resource 24. The Cisco IP Voice Media Streaming Application service must be activated in order to use Music On Hold 26. Unicast Music On Hold is the system default and functions without configuring an MOH audio source, MOH server or media resource group list 33.

Multicast Music On Hold functions only if both media resource groups and media resource group lists are defined to include a multicast MOH server, and at least one audio source must be defined to allow multicasting 31. Multicast is enabled on a media resource group by checking the Use Multi-cast for MOH Audio check box 35. Multicast Music On Hold supports only IPv4, its audio streams are not encrypted and do not support SRTP, and CTI and MTP devices do not support it 30. When an MTP is invoked in a call leg at a multicast MoH site, the Release 15 guide's remedy is to configure unicast MoH or Tone on Hold instead of multicast MoH 29.

On the Music On Hold server, Maximum Half Duplex Stream is the maximum number of devices that can be on unicast music on hold from that server at one time 23. Maximum Multi-cast Connections should be at least the number of devices that might be on multicast MOH at one time 23. When a Music On Hold server fails during an active Music On Hold session, the held party hears no music from that point 32. The Release 15 guide lists use of the G.729a codec for music as a cause of a user not hearing acceptable Music On Hold, because G.729a is designed for human speech 22.

Diagnosing it

Start by confirming that the resource is registered and that the service hosting it is running: 42 52

For conference failures, the Release 12.5(1) troubleshooting guide directs verifying that software or hardware conference bridge resources are registered with Unified CM 42.
When a hardware transcoder is installed but calls between incompatible codecs still fail, the Release 12.5(1) troubleshooting guide gives as a possible cause that no transcoder resources are registered with Unified CM 10. It directs checking the ResourceAvailable counter of the Cisco MTP Device object in the Real-Time Monitoring Tool 10.
For software MTP, annunciator and Music On Hold, check that the Cisco IP Voice Media Streaming service is running or activated on the relevant node 52 3 26.

MTP devices unregister and disconnect after a hard or soft reset, and reregister with Unified CM after the reset completes 41. If the primary Unified CM node fails, a transcoder attempts to register with the next available node in its Unified Communications Manager Group 56. A transcoder that is assigned to a media resource group cannot be deleted 55.

For Cisco IOS DSP farm resources, note that DSP farm services communicate with Unified CM using SCCP and that other protocols are not supported 12. In the IOS associate profile command, the device name given for registration must match the device name configured in Unified CM; otherwise the DSP farm profile is not registered to Unified CM 15. The IOS interoperability guide verifies DSP farm registration with show sccp ccm group, show dspfarm profile, show sccp connections and show dspfarm all 16.

Applicability

Applies to: Cisco Unified Communications Manager and Cisco IOS voice gateway DSP farm. Deployments: on-premises. Sources checked 2026-09-30. MTP, transcoder, Trusted Relay Point, annunciator, conference bridge and Music On Hold configuration details come from the Release 15 and SUs guides 38 47 24. Symptom and troubleshooting guidance comes from the Release 12.5(1) troubleshooting guide 42 46. DSP farm verification guidance comes from the IOS interoperability guide 16.

What remains uncertain

Sources disagree on software MTP capacity, with 24 calls per application in Release 12.5(1) and a default of 48 resources in Release 15, so verify the figure for your release 50 51. The default values of Fail Call If MTP Allocation Fails and Fail Call If Trusted Relay Point Allocation Fails are not covered by the sources below. The Supported MOH Codecs service parameter and its default in Release 15 are not covered by the sources below. The full definition of the MediaResourceListExhausted alarm and its media resource type values are not covered by the sources below. Release 15 lists SIP Early Offer over SIP trunks and DTMF transport mismatches as reasons to insert an MTP 38. How much MTP demand those drivers create, and the MTP Required setting on SIP trunks, are not covered by the sources below. The documented Music On Hold failure case covers a server failing during an active session 32. What the held party hears when no Music On Hold resource can be allocated at all, including any Tone on Hold fallback, is not covered by the sources below. How a conference bridge is selected through the conference controller's media resource group list is not covered by the sources below. Allocation failures for conference bridge types other than the software and Cisco IOS Enhanced conference bridges, including SIP-based conference bridges, are not covered by the sources below. Field patterns for Cisco IOS media resource registration failures are not covered by the sources below.