When I hear about "click-to-call," I think of search advertising, but click-to-call buttons are popping up all over the internet, including email and web sites. In most cases, the retailer is calling back the prospect rather than answering an inbound voice-over-internet call. Of course, this also increases the appeal to database marekters, because now they have the prospect’s phone number.

WSJ.com:Marketers Embrace Click-to-Call, 2006-Sep-7, by Suzanne Vranica

"We were looking for a strategy to increase sales efficiencies," says Jeanniey Mullen, senior director of email marketing at OgilvyOne, a direct-marketing unit of WPP Group, which is behind the Six Flag’s push.
OgilvyOne, which has been promoting click-to-call, says 10 of its clients are using the technology.