Unable to Receive Incoming Calls from the SIP trunk – Session Timer Required


  • Firmware version: Any
  • Model: S-Series, U-Series

Problem Description

When receiving the incoming call through the SIP trunk, S-series PBX reply ‘421 Extension Required’, then the incoming call fails.


The PCAP log shows S-Series PBX refused the call, then required ‘Timer’, which means S-series PBX requires the SIP provider sends ‘session timer’ at the INVITE field. 



Since the SIP provider is not easy to do the changes, then we disable the Session timers at S-Series PBX side.


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