As I was driving across the Southwest last week, I saw the signals on my collection of prepaid phones come and go. If I wanted to use a prepaid phone exclusively, which is certainly a possibility, I would want a different prepaid phone active in different locations. By using Google Voice (and similar call-directing services), I could program a different phone to work in different locations, and the incoming caller would still call the same number.
Extending that thought, I could also program a daily schedule that forwards calls to one phone, say, during business hours and another in off-peak hours, and incoming callers won't know the difference. No, I really don't want to carry more than 1 phone, but while traveling along the New Mexico-Texas border, my AT&T GoPhone had absolutely no signal, but T-Mobile prepaid worked just fine. Many Texas rest areas have free wi-fi, so I pulled over and re-programmed my incoming number to forward calls to the T-Mobile phone, and I was back in business. This also helps me on our frequent visits to Mexico.
A simpler option is to just Call Forward calls from your normal phone to the cheaper phone, but the online services can be adjusted even when you don't have a signal from the orginal carrier. Since you're using minutes on the original phone, your advantage is only realized if you have no signal on the main phone, or you would be incurring roaming charges.
Oh, my kid, who doesn't have a wireless phone, used that rest area wi-fi to Text a few messages on his iPod Touch...no phone needed. Yes, that iPod could be one more tool as well, but I can't get him to stop playing Sonic Hedgehog II long enough to make a call.