Quote Originally Posted by usa2k View Post
If cell companies can play songs to the caller, instead of ringing, does that mean they connected the call? If you get a lot of LD inbound, are you guaranteeing the caller has termination fees? I guess they must. That is revenue for the cell company each time.
Not really... Audio can be sent before a call is connected to replace the standard ringing sound. Once the call is connected, the "ringing" stops. This is called "early media".

You can tell which cell phone providers are using SIP in their backbone this way...

Here's the RFC, if you'd like to take a look: http://www.ietf.org/rfc/rfc3960.txt