Because you're not buying anything, O2 doesn't consider your call important, so use the Lost/Stolen or Fraud option if you have to, to get through. Otherwise you'll just be disconnected.
Alternatively, below is the advice from an O2 Guru on how to get through.
Call 202 from your mobile.
When it comes to the reason you are calling, stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.