Siphon is a SIP/VoIP client for the iPhone and iPod Touch that allows you to save money on calls when Wi-Fi coverage is available. You can enable VoIP on 3G/GSM, if your carrier subscription allows it. Siphon allows you to make calls trough SIP providers all over the world, using an interface similar to that of the iPhone. Unlike other applications, Siphon is a true SIP client, and connects directly to your SIP provider. Siphon uses the address book and Favorites of your iPhone.