The company has acquired six-year-old VoIP service provider Dialpad Communications for an undisclosed amount.

Yahoo will use Dialpad’s PSTN capabilities to add PC-to-phone and in-bound PC calling to its portfolio at some future point, said Terrell Carlsten, a Yahoo spokesperson. In other words, Yahoo users would be able to make and receive traditional phone calls using the Internet.

New premium Yahoo PSTN services are slated for release in coming months, she said.

Earlier this year, Microsoft announced VoIP capabilities for its instant messaging service.

While Yahoo has offered its free voice chat instant messaging feature since 1999, last month launched its first SIP-based VoIP offering, which enabled two-way conversations.

We definitely have prioritized the enhancement and extension of our voice services, Carlsten said.

Yahoo is not planning any VoIP services that will be a replacement for a primary phone line, she said.

Still, its push into the market is sure to take business away from other VoIP providers.

Yahoo also announced yesterday it has licensed a SIP-based development kit from Xten Networks Inc to beef up its PC-to-PC VoIP.

Yahoo currently is using just the voice-engine component of Xten’s software development, called eyebeam.

However, it seems likely that at some future point Yahoo may also add voice and open-standard IM to its offerings using the kit, said Erik Lagerway, COO of Santa Clara-based Xten.

Privately held Dialpad, based in Milpitas, California, has about 40 staff, most of whom will be incorporated into Yahoo’s communications products team, including Dialpad CEO Craig Walker.