Alec Saunders has an interesting concept of how a service like Google's new click-to-call might work. In fact, he's covered all the angles, including call tracking, for the advertiser and Google, long distance, etc:

-Use a VoIP application server and softswitch to manage the traffic. It's by far the least expensive to build out, and the fastest way to get a service into market.

-Write a Click-to-Call server (it could be as simple as a CGI script!) to convert those clicked links into signalling requests to call two end points on the network --- one at the advertiser, and one at the prospect.

-Route the traffic using an IP backbone provider like Level3 to a destination gateway. At the gateway closest to each of the PSTN numbers given, drop the call onto the local class 5 switch.

While Alec is quick to point out this is just how he would do it if it were him, I'm inclined to jump on the bandwagon and take a guess that this may well be how Google is accomplishing click-to-call.

We wrote on a similar service a month or so ago, that ADS-Click released at Ad:Tech. Of course, if you remember, their service utilizes Skype.