With voice lines eroding rapidly, AT&T, British Telecom, NTT and other incumbent carriers will soon launch their own IP telephony competitor to Skype, essentially writing off voice revenues entirely while moving customers to data, wireless and IPTV services, investment banking firm ThinkPanmure predicted today.

In a research note, ThinkPanmure predicted a group of 10 to 15 incumbent global operators will launch a competitor to P2P Internet telephony service Skype by 2009, with the motivation of keeping voice subscribers from “completely disappearing, reducing win-back marketing costs.

“We believe that these incumbent operators will join together in the launch of their own de-facto Skype competitor, so that they may keep as many customer relationships as possible,” according to the research note. “The service would be free when calling any other subscriber of the consortium, consisting of perhaps 10-15 incumbent carriers around the world.”

Beyond retaining those customer relationships, ThinkPanmure said incumbent providers could insist that users of the IP telephony network access it using a telco-provided broadband service, such as DSL or emerging 3G/4G data services. Operators would be able to uniquely brand their IP telephony offering and would need some level of technical coordination to ensure basic levels of service, even on a largely “best-effort” P2P IP telephony network.

“The incumbent, while losing some telephone revenue, can use the power of the DSL line to upgrade the customer to IPTV or to add one or more cellular subscriptions down the road,” the report said, while predicting that eventually wireless providers would offer free IP-based voice service as well to customers purchasing wireless data services.

Delving deeper into the numbers behind this prediction show it to be as bold as it sounds.

The challenge is that even as wireline lines are eroding, those customers still represent a large percentage of telco revenue with margins that outstrip what carriers can earn in more competitive (but growing) wireless or enterprise businesses. Consumer wireline still accounts for 29% of Verizon’s revenue and 21% of AT&T’s, according to Bernstein Research analyst Craig Moffett.

In Telephony’s most recent cover story, By The Numbers, Moffett said that while new services such as IPTV have great growth potential, they offer “little or nothing for long-term margins, replacing as [they] do high-margin telephony business with low-margin video,” Moffett said.

ThinkPanmure, which was created last year by the combination of UK investment bank Panmure Gordon & Co and US firm ThinkEquity, noted it originally wrote about this concept back in 2003.