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Sipdroid allows you to use Android phone with almost any SIP provider. The calls are crystal clear even over a 3G network, likewise for a WLAN connection. The best part about Sipdroid is that it integrates into your phone, eliminating the need for a separate phone book. You simply use your Android contacts in the same way as if you were making a call over a cellular network. It’s really an amazing application that is stable and simple to use, with the potential to save you plenty of money.
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combine Gizmo (SIP+VoIP service), Google Voice and SIPDroid on Android to make free US calls over wifi/3G