Google voice lite is what you want if you just want to use it for voicemail. Are you on a prepaid T-Mobile plan? If so it won't work as that doesn't support conditional call forwarding.
i read somewhere that if you call T-Mob and have the T-Mob Voicemail turned off that Google Voice "LITE" Voicemail will work. i personally have AT&T GoPhone and CCF and GV LITE works like a charm.
That's the case if you want to use full Google Voice (separate phone number) on prepaid T-Mobile. If you just want to use Lite, CCF is required for it to forward calls from cell number to Google Voice voicemail number. T-Mobile doesn't allow this on prepaid accounts which is a bummer.