You can leave at anytime but unless you have met your minimal contract term (if on a monthly contract) you will have to pay off the remaining months.
If you are PAYG, just call 4445 or ask live chat for your PAC and move your number on to the network of your choice.
If on contract and need clarification of charges etc call 202 from your o2 phone.
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)