New phone through Carphone Warehouse, same network as current sim only. Want to keep phone number.

Hello.

I bought a new phone through carphone warehouse and it was on Id Mobile. My current phone number is through a sim only with Id Mobile. So, not actually as an upgrade. I didn’t realise that you couldn’t just use a PAC code like normal when you’re on the same network, but desperately want to keep my old phone number. Please help. Live chat kept not loading, so I’m turning to here. 

Thanks