I just confirmed in my Wireshark that Google Call on Gmail uses exactly what I recommended in a previous post back in 2009 when they acquired Gizmo5. And YES, it's Jingle!
Their wise choice of having a portable and extensive protocol(XMPP) as the bus and having specialized technologies like SIP, will grant them a flexibility never seen before on platform and device portability.
Their master plan is to be able to delivery mass market a cheap and alternative method for calling the old fashioned telephone numbers. And sure they have the right platform and tools in their hands:
Almost all Android phones have with GTalk application pre-installed, which already runs a nice XMPP Client, besides other great alternatives like Nimbuzz.
Now imagine what Google can bring to the market without much effort due their choice of using Jingle extension of XMPP for their service? Google Call support on Android phones. That is the key and reason behind this service.