I'll see if I can make this even more complicated
Where the ringback tone is generated depends on "early media" and how the OBi device handles that. "Early media" is any audio and/or video that is sent in either direction before the call is actually answered. This might be an information message or a tone etc.
With regard to SIP the "180 Ringing" message is sent from the callee SIP equipment back to the caller SIP equipment (OBi in this case) to indicate that the callee is actually being called. The OBi device handles this as follows:
1. If "early media" has been established and an RTP path has been established between caller and callee before the "200 OK" message (call answered), then the OBi will ignore the "180 Ringing" message and pass the early media to the caller. So a voip provider may send its own ringback tone via early media.
2. If no "early media" has been detected by the OBi, then when it receives the "180 Ringing" message from the callee it will generate its own ringback tone locally and send that to the caller.
In my own setup I have tested one provider (sipgate) that uses early media to supply ringback tone, so case 1 applies. Another provider (sip2sip) does not provide early media, so case 2 applies.
Of course, GV uses XMPP not SIP, but I believe that XMPP behaves the same with regard to early media as SIP. Also, I believe that GV does not provide early media, so it is likely that it's the OBi providing the ringback tone locally. This could be proved by changing the relevant tone pattern in the OBi. However, I would say that I am very much NOT an expert when it comes to GV, so maybe our resident GV expert would like to comment on that.
I trust that's helped anyone who managed to stay awake to the end of this post
Edit: There is another scenario where a voip provider who wishes to use "early media" does not send the "180 Ringing message" back to the caller at all, but instead sends a "183 Session Progress" message which contains all the information (connection ip address, codecs to use etc) back to the caller SIP equipment (OBi in our case). This enables an audio channel to be opened for sending tones, IVR announcements etc before the "200 OK" message confirms that the call is connected and call charging can begin.