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

Digium 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:
 

To fix this issue:

1. Open /etc/asterisk/sip.conf with text editor 

2. add session-timers=refuse to the Digium SIP peer Configuration 

3. Save and Exit the editor 

4. 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.