Yes, biomesh, I believe you are correct.

I talked to an asterisk guru who said voipo uses a "forced" session timer instead of an optional/support session timer.

It was a bit over my head, but something unique about the voipo headers compared to some other providers.

There is a "session-timers" variable that can be set in asterisk to help address this difference.

I tried it, and it seems to have resolved the issue for at least one customer.

Oddly though, the problem does not exist with all calls or customers. So there's still some other unique condition that triggers it.

Perhaps a combination of which upstream carrier is being used for a particular call?