Phone has suddenly stopped receiving calls

I have recently upgraded to a Samsung 8 through ID. In the past few weeks I am no longer receiving calls. I can make them, send and receive texts no problem. I've tested calling from landlines and 2 different mobiles but none of the calls go through. It either goes to a line engaged tone or to my voicemail. I've used the call settings to disable call forwarding and it works for about two minutes, then the settings go back to what the were (the mailbox number) and I stop getting calls again. I've used the code ##21# and that works for a few minutes - I've uninstalled WhatsApp and I still have the same problem. Only now I can't register WhatsApp as the text with the code won't come through. I've replaced the SIM card as instructed by ID and yet it still isn't working. Can anyone help? Thanks