T-mobile's prepaid service offers a deal unlike any I have seen anywhere in the world. With their prepaid service, once you have put $100 of airtime into the phone (which gets you 1000 minutes if you do it at once) then that and future airtime lasts for a year.

This means you can keep a GSM phone alive, if you don't use it much, for $10 per year, after you've put in the initial outlay. Frankly, even $100 per year is a decent deal which you can't get from other carriers. (Virgin Mobile USA will do it for about $80 per year.)

So that's great for a second phone, but it's still not what I propose here, which is a second phone that works on the same account, and rings on the same number.