There has been a few issues like this recently (issues with voicemail) @Martin_P .
There is a guide on voicemail, including turning it off/on again in the link below.
Guide: All you need to know about O2 Voicemail 901
Someone else got it working by turning off ALL diverts and then turning voicemail back on - see link below.
https://community.o2.co.uk/t5/Tech-Support/Can-t-access-voicemail-1750-202-etc/m-p/1583411
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 Do not publish personal details (email, phone number, bank account).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
