Where can I get information about iOS file handlers for VOIP apps?

I have an iOS app that makes phone calls using the Telephone application开发者_运维问答 by using a tel:// file handler. I'm looking for the file handler for VoIP applications so my app can work on iPad and iPod touch. Where can I find out the file handler for Skype and other VoIP apps?

(I assume Skype is skype://... What about Fring? Google Voice?)

I'm not sure there's a simpler way than downloading these apps from iTunes, unzipping the .ipa file, and looking into their Info.plist for their CFBundleURLTypes.