You can do this by simply calling customer services on 202 (Pay Monthly) or 4445 (Pay as you go).
O2 will allow you one free mobile number change should you wish to change it.
After that you will be charged a one off fee of £35.00 if you want to change again.
Things to keep in mind:
If you call to change your number and call from the number you want to change, the call will get disconnected as part of the process. The number change is processed and a new number is auto-allocated, which disconnects the existing number immediately.
If you can call from an alternate number, you will be able to finish the conversation and get the new number from the advisor. Otherwise, you will have to call someone to find out what your new number is once it’s complete.
Change your O2 mobile number to your previous number
Alternatively, if you have a SIM card from another provider which you don't use anymore and is still active, you can transfer your number across to your current O2 SIM card free of charge by calling one of the above customer service numbers after obtaining your PAC code.
I want to keep my number, but am getting a lot of unwanted calls: what can I do?
If you'd rather keep your current O2 number but you're getting nuisance calls, you can register with the Telephone Preference Service (TPS) for free and will dramatically reduce cold calls from companies after 28 days. This service is free of charge.
Register by visiting the website or by texting TPS and your email address to 85095.
It is also worth noting that most modern smartphones have a call block feature which will block all calls and texts from a number. Settings differ from handset to handset so please check your user guide to find out how if you don't know. Once the call block is in place, the person you've blocked will be diverted straight to your O2 Voicemail. If you want to switch that service off and have O2 Call Alert instead which notifies you of missed calls when your phone is switched off or out of service then call 1710 free from your O2 handset.