Put the new simcard in and keep rebooting the phone
number transfers are going very slow and don't usually happen at weekends
If you hagve no new sim
You need to call O2 then choose any of the options, fraud etc
Everything is taking longer due to the lockdown. Call centres and social distancing means limited staff to handle all the queries. Patience needed I'm afraid.
Other numbers to try
0800 977 7337
0800 587 4005
@Tommy8 If all you did was put your old sim into your new phone there shouldn't have been any problem. I suggest you take the sim out and try putting it back in again. Make sure you've put it in the right way. Sometimes it can be something as simple as that.