Question
You have a Teams Phone deployment that uses Direct Routing on a single Session Border Controller (SBC).
Users report that outbound PSTN calls are failing, but inbound PSTN calls are successful.
When you review the SBC logs, you discover that Microsoft is responding to SIP option requests, but is NOT sending SIP option requests.
What is the cause of the issue?
- A. The online PSTN gateway is disabled.
- B. An SBC certificate is expired.
- C. An external DNS entry is missing from the FQDN of the SBC. Correct
- D. The phone numbers of the users are defined by running the New-CsTeamsUnassignedNumberTreatment cmdlet.
Explanation
When the SBC receives the SIP 200 OK from the Microsoft SIP proxy but never receives the SIP OPTIONS that the proxy sends, Microsoft documents that the FQDN in the Record-Route or Contact header must resolve to the correct public IP address; without a public DNS record the proxy cannot reach the SBC to send OPTIONS or new outbound INVITEs. Inbound calls still work because the SBC initiates those connections. An expired certificate would break the TLS connection in both directions, so Microsoft would not respond to OPTIONS either. A disabled gateway or an unassigned number treatment does not explain Microsoft answering OPTIONS while never sending its own.
