my iphone 4s diverts to voicemail without ringing - help

My iphone 4s has started to go direct to voicemail without ringing and gives no notification of a voicemail message. This has only happened since 13/01/14. I have tried a back-up and restore, hard reset, turning do not disturb on and off and reset network settings, so now I am at a loss as to what to do next. Any ideas or do i give up and throw it out the window - frustrated!! please help