Toktumi hopes to fill the gap. The San Francisco start-up sells a 99-cent iPhone app called Line2 that turns the iPhone into a dual-mode handset. In plain English, that means you can make and receive calls over the Internet through Wi-Fi or cellular, using your Line2-issued phone number. Result: You're no longer hung up if AT&T's coverage is spotty or simply unavailable, as it often is indoors, so long as there's a wireless hot spot.
Using Wi-Fi, Line2 voice quality is generally excellent, especially so if you're gabbing with another Line2 customer. On cellular, the quality is as good or bad as any AT&T cellphone call. You can also make a VoIP (or Internet call) using AT&T's 3G data network, but the quality is so-so.
You can usually detect a decline in quality when slipping from Wi-Fi onto AT&T's airwaves. Line2 is supposed to make the "handoff" from one network to another seamless, though it didn't always work out that way in my test.
While getting calls through when your cellular reception is lousy is Line2's main reason to crow, there are others. You might actually save money. Toktumi's service costs $14.95 a month for unlimited calling in the USA and Canada, which you have to pay on top of regular AT&T monthly rates. But the assumption is you'll make a lot of calls when you're near a Wi-Fi hot spot -- and those calls don't count against your regular AT&T voice allotment.
So you may be able to downgrade to the cheapest $39.99, 450-minute AT&T monthly voice plan vs. the unlimited $69.99 monthly plan.