Source note · SIP 503 and trunk out-of-service failures
If 540998/503 occurs often, Microsoft says to check that the SBC re-resolves the Direct Routing FQDNs frequently, ideally every 15 minutes or more often.
Checked 2026-09-25
Ideally, DNS resolution should occur every 15 minutes or more often.
- Vendor
- Microsoft
- Product
- Teams Phone Direct Routing
- Subsystem
- DNS
- Deployment
- multi-tenant
- Region
- not restricted
- Release range
- current as of 2026-09-25
- Checked
- 2026-09-25
Sources
- SIP 503 and Microsoft response codes - Microsoft Teams — Microsoft · tier 2 current vendor documentation · Section '540998 503 Service Unavailable - Service state: Inactive/DrainingTransactions', second paragraph
Cite this note
APA
WarmTransfer. (2026, September 25). SIP 503 and trunk out-of-service failures: source note sip-503-trunk-failures-dr-dns-refresh-15min. WarmTransfer. https://warmtransfer.net/knowledge/claims/sip-503-trunk-failures-dr-dns-refresh-15min
BibTeX
@misc{warmtransfer-claim-sip-503-trunk-failures-dr-dns-refresh-15min,
title = {SIP 503 and trunk out-of-service failures: source note sip-503-trunk-failures-dr-dns-refresh-15min},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sip-503-trunk-failures-dr-dns-refresh-15min},
note = {Source note sip-503-trunk-failures-dr-dns-refresh-15min, checked 2026-09-25}
}