In all honesty, your best bet would be to return the watch to O2 and just buy the non cellular version direct from Apple as that is the only way it could be used on a Voxi contract.
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)