It works for both computer to computer or computer to regular phone (aka PSTN) calls.
However, in order to make calls to regular phones you need to open an account at a service like
www.SIPPhone.com Once, you pay for some "SIP Minutes" you are able to make calls via the Xten softphone to PSTN phones both domestically and internationally. Voice quality has been pretty good. No monthly charges at SIPPhone to maintain your account. Computer to computer calls are free no matter if the receiving SIPPhone is domestic or overseas.