Moved back to O2, number has moved to old sim?

[Calico]

Hi all,

Put simply, I moved from O2 to another network a few years back and I've now moved back to O2 and I've had an issue with the number switching.

I'll try to simplify this:
During my first stint on O2 I had sim card #1
During my stint with the other network I had sim card #2 with my number carried over from sim #1
I'm now back on O2 and inserted the sim that was posted to me with my new phone (which I'll call sim card #3)

I supplied my PAC code and the number has transferred from sim card #2, but it's gone to sim card #1 rather than #3!

Sim  1 was for a 4G enabled phone, Sim 3 was for a 5G enabled phone - should I just put Sim 1 in my new phone or do I need O2 to switch this internally for me for the sim to function properly?

[MI5 · Accepted solution]

@Calico 

Use sim 1 for now and then pop into your local O2 store for a free swap to a 5g sim.

You need to take ID with you.

Going in store will ensure your are not cut off.