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)