Any iPhone SIP/VOIP Application that can be called from other iPhone App?

Is there any VOIP app for iPhone/iPad that can accept call request from another custom iPhone application?

I have a custom iPhone application written with C# and MonoTouch. I need to make VOIP calls (and conference call with multiple phones) from my iphone application. But developing a VOIP app from ground up is no开发者_如何学Pythont worth the time (and I dont know Obective-C either). So I was looking for an SDK or App that can accept call request from other apps. For example my app could construct and call a URL like (app-protocol:call?18002221111&18003331112&18001322244)... an installed VOIP in iPhone will listen to the protocol and place conference call to all three numbers. (Something like this is possible with skype but skype does not support conferencing in IOS... although works fine in Mac or Windows).

Thanks

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Thanks for your answer Paul. Do you know if gvconnect url scheme will support bulk call (multiple numbers at the same time? For single number I can have skype installed (Free), I tested the skype: url scheme and that works fine. The problem is it does not work with semicolon delimited phone numbers in iOS ... which works fine in skype for Windows Desktop version.

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I think GV Connect - Google Voice Connect can do this for you. It supports gvconnect:// url handler scheme. You can find a list of apps which use custom URL handlers here http://handleopenurl.com

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I haven't tested this one but it could be also helpful:
Siphon, an open source iPhone app.