Try dialling ##002#. This is the short code to cancel all diverts, including Voicemail 901.
Note, however, that any other diverts you've set up will also be cancelled and will need to be reinstated if required.
Alternatively, you could try activating O2 CallAlert, which is an alternative to Voicemail 901 and will automatically replace the 901 service.
To activate CallAlert, dial 1710. For more details, click
HERE.
