Very Weak or No Signal

When trying to make a call, I get the error "Not registered on network". Sometimes if I move to a different area of the house I will be able to call out, but the call drops while dialing, never connects, or drops shortly after connecting. I have tried all the troubleshooting methods (rebooting, checking the SIM card, airplane mode, resetting network, no network outages) but still cannot get it to work. The strange thing is that I'll get a "new voicemail" message without my phone ringing. I cannot make phone calls to get support so I guess this is the method of communication. Can someone please help?

Thank you,

Lisa