To get the old number you have to cancel the contract (can only be done by the person who is named on the contract). If it has reached the end of the minimum term this is no problem but if not you would have to pay up to the end of the contract to end it.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)