Has anyone else had issues with not receiving calls since adding one number?

After buying an Apple Watch 5 Celluar I have had big issues where neither my iPhone X nor watch will receive calls but can make calls, send/receive txt and use data.

I think this may have something to do with Hi Voicemail which uses call forwarding however even after deactivating their service I'm still receiving txt messages saying I missed a call from someone when the phone didn't ring. Calling myself off my landline goes straight to voicemail.

The last time I had this issue I spent days getting tossed around the call centre and live chat with little success which I appreciate was probably due to it being such an odd issue.