Yahoo yesterday announced that it’s no longer going to power its IM client’s Internet telephony service with in-house technology. Instead the company has reached an agreement with Jajah by which Yahoo Messenger’s Phone In and Phone Out features will run on the Jajah service.

Under the agreement, Jajah will connect calls to and from Yahoo Messenger starting in the third quarter of 2008 as well as take over all aspects of customer support and billing. 97 million users supposedly use Yahoo Messenger – though it is not clear how many of them use the internet phoning features – so this is definitely a big opportunity for startup Jajah, which has managed to build up a customer base of its own of 10 million customers. If the Microsoft-Yahoo merger goes through, however, it remains to be seen what the implications for Jajah would be, as the software giant may want to use its own voice technology with Yahoo Messenger instead.