I’ve run into this twice now on two different routers. The Sonicwall TZ170 and another Zyxel model.
On an intermittent basis, outbound calls that route through these firewalls (and probably others) would simply drop after 30 seconds or so of successful two way audio.
the issue turned out to be a default UDP timeout on the router. Changing the default from 30 seconds to 90 solved the problems.
Interestingly, with one of the Sonicwall Tz170, after a power drop and reboot of the router the default setting was replaced, causing the problem to resurface. Once this was once again changed the calls were no longer dropped.