Phone divert not working

I have an iPhone 15, with up to date software. For many years I have regularly set a divert to my assistant's phone number when I am in meetings or on holiday and cancel the divert afterwards. My carrier is O2. I use the following dialling codes:

To set divert: **21*[phone number]#

To cancel divert: ##002#

These no longer work and result in this error message: "Error performing request: unknown error".

The Apple Helpdesk cannot find a problem and referred me to my carrier, O2. O2 have not been able to come up with a solution. Meanwhile, I can't cancel my divert and all my calls are going to my assistant!

If I dial the code *#21# - check divert status, it says "Setting interrogation succeeded. Voice call forwarding on all calls disabled." Which is nonsense, because my assistant is getting all my calls.

I would be very grateful for any suggestions.