I had the same problem for more than a week. Seems to coincide with 3G shutdown. I had a samsung J6 which I thought was 4G enabled but it turns out voice calling was being achieved using 3G and that was revealed once 3G was gone. So I bought a new Moto G35 5G for £98 and I can now send and receive voice calls. Now when I look at the signal strength, there is a little "4G" grapic next to it which wasn't there before. This phone will handle 5G with some providers and it has an option for wifi calling. Hopefully I am future proofed for a while.

The exact same has been happening to me today. 

I can receive texts and dial out from my phone but any incoming calls are met with a number cannot be recognised. 

Exactly the same thing has happened to me today. Smarty say it could take 3-4 days to look in to. Extremely frustrating