Yahoo is outsourcing the internet phone functions of its instant messaging (IM) programme to Jajah, an Associated Press report said.
Jajah will connect the calls to and from users of Yahoo Messenger and handle billing and customer care, the company said.
Terms of the deal where not announced. No employees are moving over to Jajah.
The deal is an important win for Jajah, which has grown rapidly since it was founded in 2005 in Austria. It is now based in Mountain View, California, and has 10 million customers.
Yahoo Messenger has 97 million users, but the company has not revealed how many of them use the premium Phone In and Phone Out features.
The Associated Press report quoted Jajah CEO's Trevor Healy saying that users will see no change to the service.
The Jajah brand may appear in some places, along with Yahoo's.
Privately-held Jajah said it hopes to attract more corporate customers like Yahoo with a package of managed services for voice calls, and is talking to phone and cable companies.