Bad service

For the last 2months i have Been having problem with making calls to other cell phones. Will Dial the No. then it will cut off or say the no does not Exites. Some times i need to Daile the no 3- 4 time before i get through to the no. People will try and call me and the calls me and the calls will not come through. Ill also call customers and it will ring by me but not by them.