Sticking it to The Man just got a little easier with Skype for iPhone's addition of multitasking support and elimination of plans to charge for calls over 3G. But can the ability to receive Skype calls any time really save you money? Let's look at the numbers and find out.
For People Who Don't Talk Much
AT&T won't let you get an iPhone data plan without voice minutes, so at the very least, you've got to pay $40 per month for 450 minutes. If you're not routinely talking for longer than that, Skype probably won't help you. Still, consider having long phone conversations over Skype just to reduce the chances of going over 450 minutes. This will help you collect rollover minutes as well.
For People Who Talk a Lot
Things get interesting when you regularly exceed 450 minutes per month. To truly replace your expensive voice plan, you'll need both a Skype subscription and an online number (formerly SkypeIn), which any phone can call.
A subscription for unlimited U.S. and Canada Skype calls costs $3 per month, and the number costs $12 for three months after a 33 percent discount for having a subscription. By committing to longer subscriptions, you can reduce the price even further: A full year's Skype subscription costs $30.48, and the discounted rate for a year of online number service costs $30.
Long story short, you can pay $60.48 per year, or roughly $5 per month, for unlimited calls to and from your Skype number. So instead of paying $60 for 900 AT&T minutes or $70 for unlimited minutes, you'll pay $45 for a basic 450-minute plan and Skype.
But there are concessions. You may want to set up call forwarding through AT&T or through Google Voice, sending all of your calls to Skype. Otherwise, you'll have to instruct people to call your Skype number instead of your actual iPhone number.
Also, you'll be dealing more with AT&T's data network. Not only is it less reliable than voice, but for some customers it can become more expensive. AT&T recently stopped offering unlimited data plans, so unless you're grandfathered in to the old plan, the 2 GB monthly data allotment could be a problem. That depends on how often you place calls from Wi-Fi hotspots.
For the International Talker
I can't really do the math for international calls, because there are too many variables for where you're calling and how often. Generally, Skype's international calling rates are cheaper than AT&T's, but you can compare both for the countries you call. Your monthly cost will depend on how many countries you plan to call, and whether you want to call mobile phones or just landlines.