Android Voicemail Settings

Voicemail on my T-Mobile Galaxy Portal/Spica/i5700 doesn't work. I've set it up via the Tmobile voicemail setup, but if I call my mobile it just rings and rings until I get a message saying 'This number is currently unavailable.' Is there anything in Android itself that might have borked it? I'm running an *ahem* not official Samsung version of Froyo. In Settings -> Call Settings I can see Voicemail service, from which my only option is My Carrier, and Voicemail Settings, and all that lets me do is pick the voicemail number to call. Or is it more likely just some error at T-mobile's end? Thought I'd check before giving them a call.