[macin]

I read somewhere that if I unlock the iphone using ziphone and then insert a tmobile card the visual voice mail wont work. is this true?

[analogika]

Visual voicemail depends ENTIRELY upon the network investing in HARDWARE that processes voicemail into a certain audio format and then pushes it over the data network onto your iPhone as an audio file that you can listen to at will.

No network that doesn't officially sell the iPhone will offer visual voicemail, as there is no incentive at all to build this infrastructure for a device that isn't legally available.

So: It's true.