Polycom phone hangs up after 30 minutes

i should know the answer but am drawing a blank. here is the scenaro.

1.815.210.58-1 - polycom 320 phones, vitelity sip trunks,

1 polycom 320 dials into a conference (one of the 800 number services)
2. user presses mute
3. listens to the conference, never takes the phone off of mute
4. about 30 minutes into the conference the phone hangs up

i have just started looking at this, but because it is pretty consistent i suspect there is a timer somewhere. i don’t know yet if it is the phone itself hanging up or if the firewall is causing it or if there is something in the trunk setting. the trunk uses ip authentication. if she talks periodically then it works fine.

thoughts anyone?


and will update this once I have some news