Anther use for an old iPhone (or any Driod phone that can connect to a Wifi) is to use a program like Line2 (
http://www.line2.com/ ) to make VOIP (Voice over IP) calls over the WiFI instead of paying for a contract with a wireless company. We use Line2 on our iPods/iPads for all our texting and calls over our Mifi's. Line2 is free for 30 days, then cost $9.99/mth (discount for longer terms) for unlimited voice and text.
We also have an App called TextFree (
http://www.pinger.com/content/home.html ) which gives you unlimited free text messaging. To get free minutes for voice calls all you have to do is download an App or two and try it out, then delete them. Takes about 5 minutes to "earn" 1.5 hours of voice call time. Or you can purchase voice minutes for $9.99 for 500 minutes.
We also have Skype (
http://www.skype.com/intl/en-us/home ) accounts which allow VOIP calls for about $3.00/mth.
Of the three Skype is the most reliable and highest quality voice calls, but the Line2 is catching up. Occasionally Line2 or TextFree's servers will be "down" for a few minutes and we switch to Skype. Between the three we have never had a problem making a call. Call quality is acceptable on all three, just a little clearer on Skype.
If you are around a MiFi or Wifi service that is already paid for, these apps and an old used iPhone, Droid, or iPod make for inexpensive cell phones.
If I did not need texting ability I would simply use Skype and not worry about it, but skype does not provide the ability to receive text messages yet.
For a teen (whatever age that teen is -
) that loves to text, TextFree, a used Droid or iPhone and a Virgin Mobile Mifi is the best deal going. Unlimited voice and texting and web browsing all for $40/mth (for the Mifi service) & a max of $10/mth for Line2 (free with TextFree). If you have Wifi at home, then you don't need the MiFi and the service is even less - $10/mth for Line2 or FREE for TextFree.
Frank