Not sure how to go about it on T-Mob or even if the text notification is available, but I'd be surprised if it wasn't.
On voda, I rang the voicemail number, selected 'voicemail setup', then 'notifications', then 'Text notification'. job done.

on my old 5800XM I used to get a persistent 'new voicemail' icon on the main screen, but I guess the N900 doesn't have the facility. Maybe later...

WS