Remove number from imessage

So I’ve recently switched network from EE to voxi and it took a few days for my number to be transferred over and whilst that was happening the number that was attached to my new SIM card has become the number that my iMessages go through but now my correct number has transferred I want to delete that number so all my text and iMessages are received on the same number.

I’ve contacted voxi and there instructions were no good and they suggested I come here.

I’ve tried the instructions online that Apple give, I’ve tried taking my SIM card out and putting it back in and I’ve tried turning my phone on and off and nothing’s seems to be working.