7200 system with pri phones are set to forward all it does forward the call, but the caller hears nothing on their side it just sounds like the call didn't go through. the external number will ring 1 time, and say missed call if the caller stays on the line it will ring the cell phone again 1 time and say missed call it will do this a couple of times, and then the caller will get a recording saying your call didn't go through. i have the call forward external time set to 1 sec.