You may have a spend cap or a premium rate number block on your account.
Give O2 a call to check.
Advice from an O2 Guru to get through:
If you call 202, 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.
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
Weird things happen sometimes.
Raise your cap to £5.00 to see if it helps.