If you're referring to Windows Live Messenger, here is what Microsoft says about phone functionality:

"Call your friends' mobile or landline phones.*  It's affordable, both within the US and internationally.  And calling someone's PC is always free.

*The ability to call telephones is a pay service provided by Verizon Web Calling service. ..."

For PCs, you can also have a video conversation with a webcam.

(Windows Live Messenger: Overview; see also Features)