Can't make or receive calls

Since last month I've been having trouble with making and receiving phone calls on my contact number most of the time when someone phones me it go straight to voicemail or it says the subscriber is not available I sometimes get phone calls but it's rare when I try to make a phone call it keeps saying unable to connect no matter what number I dial I contacted your call center and we that troubleshooting we even cancel all call forwarding and I'm having the same problems I've tried my SIM card in different phones and it's the same problem