Which raises the issue of forgoing cell service altogether and using only a VoIP app. You could then use an iPod Touch, an entry-level iPad, a deactivated or SIMless iPhone, Android phone or Blackberry... there are many such options. The setup procedure would generally follow my instructions for setting up 3CXphone on an iPhone.
A Few Issues to Consider
Obviously, a good Internet connection is fundamental. 3G can work but generally WiFi is needed, especially when roaming. Unfortunately, good WiFi connections are in increasingly short supply in hotels and coffee-shop hotspots, whose WiFi networks are besieged by guests streaming video. (Netflix has gone from nowhere to 30% of overall bandwidth consumption... and rising, and it's worse where lonely/bored travelers congregate.) But if your peregrinations take you to coffee shops and hotspots with good WiFi service, this can be a real alternative.
Testing the local WiFi capability prior to making an important call can help prevent frustration. You could make a test call, or Visualware offers a detailed free app for measuring bandwidth characteristics from your iDevice, iNetQCheck. There are others, such as Speedtest. Unfortunately, none of the available free testing apps currently test for VoIP-specific network performance. This can be better than the generic/overall performance (if the router has good Quality-of-Service (QoS) technology which prioritizes the real-time data streams VoIP relies on) or worse (if the network has VoIP port 5060 blocked!). My own home WiFi network performs much better for VoIP than a generic test would suggest, presumably due to the good QoS engine in my D-Link DIR-655 router, which prioritizes port-5060 traffic.
In the case of port blockage, your VoIP provider might provide alternative ports to try. Voip.ms' alternative ports, per their online documentation, are 5080 and 42872. But, these ports will probably not be prioritized by routers' QoS engines, so it's best to use them only if 5060 is blocked. (I'm advised that a VoIP-specific testing app is coming to the App Store-- great news. I'll mention it in an update here when it arrives.)
For me, with my travels, a conventional cell phone account is necessary, and using a VoIP app on my iPhone is a wonderful supplementary capability but not a wholly sufficient alternative. But if my situation were more urban and involved less travel, going cell-less as well as landline-less could work. As to the economics of it all, axing a cell line is even more compelling than unplugging your landline. Consider the following, from the Wall St. Journal:
Consumers currently pay about $92 a year on average, or 16.3% of their total bill, in taxes as part of their cellphone plan -- the highest amount ever, according to the Heartland Institute, a public policy research organization. That's up from a 14.1% tax rate in 2006. ...taxes account for 16.3% of cellphone bills on average, compared to 7.4% for other goods and services... This year, for example, some municipalities raised taxes by 3% to a whopping 75%. The wireless industry says its consumers are unfairly targeted. "Many of these state and local taxes and fees are levied against our consumers because they're good bill payers," says a CTIA spokeswoman.... residents in at least five states, including Florida, Illinois and Washington, get hit with rates that account for more than 20% of their wireless bill, says [economist] Scott Mackey... In Baltimore, for example, taxes and fees account for 27% of the average customer's wireless bill, while in New York they account for about 20%. [Wall St. Journal]...And that just regards the taxes. Cell plans themselves aren't cheap, especially if you travel. (And is anyone else enraged by that bit about cell users being ripe for targeting because they pay their bills? That's neither progressive nor regressive taxation nor a valid user fee; it's fleecing the easy mark, and it's wrong.)
There are other VoIP apps that include their own VoIP service that can make and receive calls from any phone, such as TruPhone, Whistle, Fring, FooCall, iCall, OBion... And of course there's Skype. An attraction of the app+service approach is that you're spared the complication of separately setting up the app and an account at a VoIP wholesaler like voip.ms or CallCentric. Many such apps are free to try, so you can see if they meet your needs. Keep in mind that such services might operate on IP ports other than 5060, so they wouldn't enjoy QoS acceleration if present, meaning more variability in quality from call to call and location to location. (Skype, for example, uses a randomly-assigned port to prevent blocking by ISPs.) And some operate their servers in places that might be physically remote from you, reducing quality.
In my case, my prerogative was to replace my home's landline, so the ability to roam around the world for cheap with an app on my iPhone was a delightful bonus. The Roll-Your-Own-VoIP approach was the best for my situation, but your needs might differ. Fortunately this is the dawning of the golden age of phone-service alternatives-- jump right in!
VoIP over 3G
If you have the bars, using 3G rather than WiFi can be attractive in some circumstances:
- For iPads and other non-voice devices with a 3G data subscription, and
- For international calling. For example, I've used my VoIP app on my iPhone to call from the U.S. to Europe. Thus I use data bytes rather than expensive international-call minutes. Of course, data bytes are in finite supply; your economic mileage will vary depending on your plan and usage.
One more thing: A correspondent asks about MagicJack. I tried it. It's not terrible, but I don't recommend it. It does have the advantage that you can keep using your old-timey plug-in-the-wall-type analog phone. E911 is included, which is nice for landline-replacement applications.
And how about Ooma. This was another possibility for my landline-replacement project, but Roll-Your-Own-VoIP was considerably cheaper to implement, and it required no locked-to-the-provider proprietary adapter. I'm happy, and delighted with the flexibility.