Line2 iPhone VoIP app lets you make/receive calls via Wi-Fi/3G
The Line2 iPhone app from Toktumi is a VoIP calling application that lets you use Wi-Fi and 3G networks to make or receive calls with your same telephone number. Line2 may be particularly interesting to international travelers, as they can make calls back home without incurring international fees. On the flip side, making international calls starts at $0.02.
Line2 is so named because you actually get a second line on your iPhone replete with voicemail, caller ID, etc. Line2 costs $0.99, but monthly service runs $14.95/mo with unlimited US & Canada calling.
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More info from the manufacturer
Price: $0.99 for app; $14.95/mo for service
(Please note prices are subject to change and the listed price is correct to the best of our knowledge at the time of posting)