freeswitch/libs/freetdm/docs/glare.txt

Glare is a PITA.

Although configuration of ISDN links can be done to minimize glare, we also must have a clear policy
on how the FreeTDM API is supposed to behave on glare across signaling modules.

There is a well-known race in the FreeTDM API since the beginning. When a user wants to place a call with
the old APIs:

1. ftdm_channel_open_xx (to hunt the channel by group, span or select a channel individually)
2. ftdm_channel_call_place() to place the actual call.

Since the user has no access to channel locking, between opening a channel and placing a call, an incoming call could be
received. Therefore things can get complicated and that is why you should STOP USING those 2 deprecated functions.

The replacement is ftdm_call_place().

ftdm_call_place receives the information required to hunt the channel and place the call.

If glare is detected after placing the call and the signaling stack decides to drop the local call, a regular
SIGEVENT_STOP will be sent with the hangup cause FTDM_CAUSE_REQUESTED_CHAN_UNAVAIL.

ftdm_call_place may return FTDM_BREAK if glare is detected while placing the call, in such cases you can try
calling ftdm_call_place again to hunt another channel.