Well, there are different causes to such messages. Firstly let me know following things..
1.Is it a cell or site or bsc or msc or network related problem ?
2.Do you get a display on your mobile phone of “call failed” or Do you get a message “The subscriber is not reachable” ?
3.Is the message “call failed” is just on the screen when you press the call button ?
4.Have you done the drivetest in that area ? what are the observations from l3 point of view ?
Let me know your comments.