A variant of a common email to people with Sonicwall firewalls.

Good Afternoon! I'll call in a minute as well.. what we see at is consistent with what we have seen at other locations with SonicWall and similar advanced firewalls. We can initiate an outgoing fax call, seems to be working well, until at exactly 1 minute and 6 seconds (very consistently) the UDP stream is blocked effectively hanging up the outgoing call. Mostly likely a 60 second timeout somewhere.   This is often due to rules for SPI (Stateful Packet Inspection) or DDoS protection, dropping a mostly uni-directional UDP packet stream at a certain timeout threshold.  For Sonicwall's specifically, the basic things to check are:

• Turn off “SIP Transformations” aka SIP ALG.
• Enable “Consistent NAT” with a timeout of several minutes. (likely problem)   
• Set “SIP Media” and “SIP Signaling” timeouts to several minutes. 

Helpful links:

• https://wiki.ring-u.com/wiki/doku.php?id=fax
• https://www.sonicwall.com/support/knowledge-base/how-to-troubleshoot-common-voip-issues/170503552140480/