How can i prevents cut off calls

i have installed the last version of astirisk now and configured a digum card with 4 FXO ports 2 of them are connected to a tellular the in out put gives 2 operation mobile lines and the other 2 FXO ports are connected to 2 ordinary PSTN lines… the problems occuers when any local extinsion dials to a local mobile number through any of mobile lines connected for the digum card:

  1. first the call cut off (line hung up) after defferent periods of time.
  2. second after the extinsion hung up the call the mobile line remain busy even after the call finish untill manualy i pull the mobile line wire from the FXO port then plugit back again.

this problem is just on the mobile lines but with PSTN line connected to the same diguim card everything is working smoothly…

if any body has any idea of what maybe causing this …please feed me back