Coverage & Network.

Hi,   I bought a new phone from Carphone warehouse - Google pixel 9. I was on the Three network, and I gave over the code to keep my number, when I took out the contract.  The switch was supposed to happen on Tuesday.   On Wednesday morning (today) 10am I received an email saying the switch had been completed. I didn't receive a new SIM, so assumed my old one would work in this phone. But I have no service. It says, emergency calls only.   I've tried upwards of 20 times to speak to someone on the chat function and it says 'sorry I can't chat right now' each time I try to get transferred to an agent. I can't access the app either because it's trying to send me a text and I can't receive it.    Any advice?    Thanks, Leigh