SmartPhone: VoIP & T-Mobile

I just ordered my 1st SmartPhone (No Contract) and plan to use T-Mobile.

Does anyone here use VoIP on T-Mobile's $30/month No Contract (100 minutes Talk, Unlimited Text, Unlimited Web (Throttled After 5 GB))? If yes, is VoIP close to Talk quality and is VoIP reliable?

When I sign up for T-Mobile's Service, I plan to select one of the following No Contract Plans:

1. $30/month No Contract (100 minutes Talk, Unlimited Text, Unlimited Web (Throttled after 5 GB))

2. $50/month No Contract (Unlimited Talk, Unlimited Text, Unlimited Web(Throttled After 100 MB))

Thanks in Advance.