Ringing heard by callers

Users of my phone system are reporting a couple issues:

1 - if they are already talking to someone on the phone, and a second call comes in (they are part of a ring group) the person they are talking to is able to hear the ring on the line interrupting the conversation.

2 - occasionally when the above happens, the interrupting ring will literally hang up whatever phone call they are on.

This is using FreePBX on asterisk, with aastra 9143i handsets.

Anybody? I think the call dropping might be due to trunks getting used up (currently have 5, bumping it to 6), but the ringing heard by callers is still up in the air.

Where does one set the behavior of what to do when the trunks are full? IE is there a way to have it finally present a busy signal or will it always cause these interruptions/dropped calls?