SIPStation and FAXStation - How do I fix Unknown refresher warnings and drop calls related to Sangoma SIP trunk connections?

Sangoma SIP Trunking servers does not support SIP Session Timers therefore any VoIP system with this feature enabled may see warning/error messages and even experience drop calls.

In Open Source Asterisk, this issue is indicated by the following CLI message:

WARNING[XXXX][C-XXXXXXXX] chan_sip.c: Unknown refresher on...

To fix this issue:

Open /etc/asterisk/sip.conf with text editor
add session-timers=refuse to the Sangoma SIP peer Configuration
Save and Exit the editor
Reload your asterisk configuration.

if you are using FreePBX add the session-timers=refuse line to /etc/asterisk/sip_general_custom.conf

For any other third party VoIP system, please seek assistance from your system manufacturer.