i have a GSM adapter plugged into a fxs port.
If i place a call from a IP phone everything works fine. If i put on hook the ip phone and try to place another call i get a busy tone and the only way is to unplugs and plugs again the cable that connect the fxs port with the gsm adapter.
I suppose it's a signaling problem
here is the voice port config
timeouts call-disconnect 3
timeouts wait-release 3