munchkin: If you ring your own mobile number, and push * when you hear the message telling you to leave a message, you'll get to your mailbox - I do this and it uses my included minutes rather than being charged $0.20
I set the number up in my contacts as: +6427xyznnnn (pause) * (pause) [voicemail pin code] and then I get there straight away without any fussing around :)
Tried this and it worked,.. but later on when I checked my account there was a charge which I think was for voicemail, even though I have a lot of free minutes remaining. I'll check again next time I need to use voicemail