So, I got home (to the US) a few days after the 6 month limit. The phone connected to the US network and I got the Rate Advice from Three. However, I could not make calls or send texts. I was able to log on after it sent me the 2FA message.

I could also get text messages but calling that number did not work - it seems the called was charged but the connection was never made.

I called Customer Service (+44 7782333333) and the guy told me the account was still active and I did have a chargeable event of 15p. So, I guess the number is still good. We will see.