I should probably clarify for the sake of argument why I'm not a fan of VoIP over wireless. The biggest argument is thus:
it's simply less reliable. This one statement can unfold into a
massive thesis, and is on my short list to do just that over at the blog.
I do VoIP in my communications setup, and I even
forward VoIP calls to my cellphone, but I
do not use native VoIP over cellular networks.
If one actually
needs mobile minutes,
pay for them. The humorous thing about your $30 price point and approach to service wanting to be frugal about it is that if you can go GSM carrier (either T-Mo or AT&T network exclusively) and either need no, limited or slow data, you can do "unlimited" talk and text plans through
Spot Mobile ($25, no data - T-Mo),
Airvoice ($30, 100MB 3G data - AT&T), or
GoSmart Mobile ($35, "unlimited" 2G data - $35) for nearly the same cash. Using WiFi at home and work, eliminating streaming and social media and using offline GPS eliminates the need for any significant data usage, and either unmetered 2G data or 100MB of 3G data a month should be plenty for email, occasional light browsing (preferably with image loading disabled) and SMS replacement apps like XMS, Kik or Nimbuzz. None of the caveats of your approach for right around the same price point. The only disclaimer I would add is that it would probably be best to not exceed about 2500 minutes (over 40 hours a month) with any of those plans just to err on the side of caution.
Is data useful? Sure, but it's not a magic replacement for voice service with mobile phones.