Thanks for your reply, I've done as you suggested and got my actual voicemail number.
I'm not sure how that helps me though......
My issue is that I cannot change the number in settings via settings>call>voicemail settings. When I do change this it say's it's successfully changed but it hasn't. Every time someone leaves me a voicemail my phone tells me to call 901. I tried replacing it with the actual voicemail number, but no joy.
Surely there must be a way of changing this, even if it requires a hack.