I've hit the same wall. I'm on a Sony Xperia 5 which has worked fine since June 2021 - which in theory according to Three's device support tool should be fine - but since Thursday night, no calls in or out and no data connection. First support person sent me a new SIM, arrived today, no difference. Tried a Huawei P9 I still had lying around, also nothing. Husband is also on Three, but different phone model, no problems, so it's not the network - walking down the street some distance doesn't help either. After a 'support' session lasting nearly 2 hrs, I am no further than I was to begin with. Well, I have been told to wait for up to a *week* to see if the problem goes away (and I thought that was the advice for a common cold, not telephony...) and that if I wanted I could cancel my contract. Yeah right... talk about the lazy way out.
At no stage did the 'agent' suggest it might be related to the age of the phone but having dug around on the internet, I suspect that there's a selection of devices which for mobile operators fall into the 'who cares' category and are somehow collateral damage in the 3G switchoff even though technically they oughtn't be.
Since the fingerprint reader got fried in the last Android update Sony pushed for my Xperia 5, I've bitten the bullet and ordered an Xperia 10 VI ... I'll let you know if that fixes the issue.
