Re: Anyone use Skype/Voip or Vonage instead of a cell phone?
how about going unlimited mobile minutes and dropping all landlines, lowering cost a different way? you can only really lower costs by lowering your minutes or pooling lines into family plans (spreading the cost across multiple people).
The only "cheap" solution is to wifi hotspot it where you do everything mobile from a laptop, including calling. other than that you won't save $$$ just look up the plans from the major carriers. You could do the cutrate pageplus/walmart or net10/target solution with dumbphone, many minutes and unlimited or many MB of mobile web and email, but for two lines you'll still pay quite a bit and need to verify coverage.
skype & vonage requires data/internet (no miracle savings). skype is horrible for voice over 3G (data), plus it sucks up data requiring more expensive data plans. Forget "4G" (ie. 3.5G in reality, if that) as that offers no benefit and higher cost/surcharges. As you know Tmobile drops down to her 2G/Edge knees faster than a well... er.
Once you NEED mobile data your options are limited as are cost savings. The best realistic & reliable way to cut your $ is to go with a lower minute plan and possibly less data (tmobile doesn't have tiers for smartphones, whereas at&t has $15/250MB month).
As evil as they have become as a corporation whose mantra is "do no evil", you can use google voice for unlimited outgoing calling if your carrier has friends&family/a-list free calling favorite numbers. Unfortunately Tmobile doesn't have "friends and family" or "a-list" so you're stuck there, neither does sprint. Also verizon/att don't give favorites with plans for less than 700 minutes (cheapest plan gets not favorites thus no googlevoice). This goees back to the original point of there is no real way to cut costs significantly below what you're paying.
tmobile does allow voice/data off contract on a smartphone like nexus one, but you can't activate said smartphone without data. You'd have to be off-contract and change the sim card from an established tmobile dumb phone without data. Then your phone and wifi would work on the N1, without data costs.
i've used vonage 4 years ago and have one line still for a business but i've transitioned away to ISP provided triple-play accounts (voice, data, tv). For mobile it's all about pooling lines & family plans (even under a smalll business account) .. glen