For T-Mobile prepaid, there are 2 options now (subject to change on/after Oct. 17, 2010) PAYG and PBTD and both are inexpensive, especially for occassional users interested mainly in voice calls, current rates for SMS are okay but no data option. The new plan being rolled out will have different price points & data caps, etc. - so stay tuned on the 17th.

Pay As You Go is more expensive but with per minute rates as low as 10 cents a minute, nationwide calling based on a $100 startup fund, achieving instant Gold Status with all unused minutes valid for up to 1 year. Pay By The Date is on days where calls are made/received, $1 fee is charged plus 10 cents a minute, plus unlimited 7 PM to 7 AM calling (plus unlimited 24/7 T-Mo to T-Mo calling)

For T-Mo prepaid, a $10 refill every 90 days will allow you to keep the initial assigned mobile # active and rollover any unused balance for another 90 days on the refill date, so $40 will allow you to keep the same mobile # active for 360 days, plus on initial (new) Sim card activation, you get a starter balance of a little over $3 USD for free, good for 90 days before the first $10 refill is required to maintain the account in good standing. If you buy (3% to 10% discounted) refill credits online with a # of reputable vendors online like callingmart . com and add them OTA while you are here, or online after setting up the account on the web, you can maintain the account abroad & will have a working mobile # on subsequent return visits to the US (for about $50 USD a year, without Gold Status by not spending $100 up front)

Also, T-Mo's prepaid allow for roaming calls in Mexico & Canada in addition to allow for international calls at varying rates, but with the upcoming data plan additions - Google Voice might make sense at very low rates depending on where your calls are going oversea using a properly configured smartphone (unlocked, quad-band device recommended AND dual-sim ones even better for today's busy travelers)

Checkout the prepaid forum section over at howardforums . com for detailed info on T-Mo's current/upcoming plans.