Once upon a time I unlocked my iPhone 3GS and used it on T-Mobile in order avoid AT&T's more expensive service.
It was a decent experience, but two noteworthy flaws marred it. Number one; I obviously had no 3G-network access since the iPhone doesn't contain the circuitry needed to piggyback T-Mobile's 3G network. Number two; I had no access to Visual Voicemail.
Of course there's no way to fix problem number one, that's an inherent issue regarding hardware. Problem number two, though, is fixable.
In this video tutorial, I'll show you how to use the jailbroken version of YouMail to get real visual voicemail on your iPhone for free...