Video call to POTS disconnects after 3 mins

Hi All,

This one has me confused.
We have 3 separate systems running Freepbx v12.0.76.2.
2 of these systems have “Video Support” enabled in SIP settings with h264 set as the only codec.
One has video disabled.

On both of the video enabled systems, placing a call via the outbound trunk to a regular POTS number, connects and voice works (no video obviously as its a POTS number), but after 3 minutes the call drops.
Every time, the logs indicate:

[2015-09-25 19:50:05] NOTICE[2798] chan_sip.c: Disconnecting call ‘SIP/Engin-00000001’ for lack of RTP activity in 31 seconds

This happens without fail every single time anywhere from 3 minutes, to 3:30.

On the system with video disabled, the call doesn’t drop.

Disabling video on both other systems fixes the problem.

Any thoughts on how to fix this?
We need video enabled as both systems have an intercom at the front door with video.
We don’t need video on outbound calls (ever).

Perhaps there’s some way to disable video on all outbound calls?