Oh my. It sounds almost like the server-side code that is doing the RFC4028 stuff (session timers) is still running, and tears down the connection when no reply comes back. If it is simul-ringing your voipo adapter and the cellphone, it's possible (maybe?) that the voipo server starts the session timer. When your ata doesn't answer, but your cell does, one would think the tiimer would be cancelled. Or maybe it is erroneously starting the timer in the first place. Sounds like a ticket is definitely in order...