Internet Telephony 
A category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls. For users who have free, or fixed-price Internet access, Internet telephony software essentially provides free telephone calls anywhere in the world.

 

What do I need to take advantage of Internet Tekephony 
  1. Inernet cnnection.
  2. IP phone or ATA (Analog Telephone Adaptor) or a softphone. 
  3. A VoIP porvider.

 

Issues
To date, however, Internet telephony does not offer the same quality of telephone service as direct telephone connections. Sometimes NAT (Network Address Translator) may cause problems.