Where can I find a comprehensive list of VOIP uris for iPad?

As an example, the skype iPad app provides handling for uris of the form:

• skype:开发者_开发问答+11235551234?call

Since there are multiple VOIP iPad apps, I'm wondering if there's a list of other uri schemes such as for fring, google voice, truphone, etc.

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The most widely used protocol in VoIP is SIP, and URIs look like this:

• sip:3333@sip2sip.info