To use your PC as a telephone other than using Skype you will need VoIP(Voice over Internet Protocol) software and a SIP (Session Initiated Protocol) account with a SIP provider.
Many offices are now using IP telephones which then have the calls fed into the national telephone network via their broadband link and SIP 'lines'.
You pay for your calls which are billed by your SIP provider.
I have on my desk an IP telephone connected via a Virtual Private Network to a remote site. If I wish to make a landline call the call travels via my ADSL Router across the VPN to the remote site where the server on that site stuffs the call out via an ISDN line!
Telecomms is an extraordinarily complicated and fast moving subject.