Voicemail stopped working after contract change

[Ian1965]

I was on pay as you go with o2, went onto a contract a couple of months ago, but since then my voicemail doesn’t work. I keep having messages to say I have a new voicemail but there is nothing there and if I try to call voicemail I can’t get through 

[jonsie]

Call your number from another phone and leave a voicemail message

Dial 901, listen to the message and delete it

This will clear the messages you keep getting

Guide: All you need to know about O2 Voicemail 901