As i understand it Skype (or messenger etc) allow you to talk, or text/message, computer to computer for free [if both parties have the same software].
With Skype (or messenger etc) you can call land lines at a certain rate per minute also.
VOIP is used to call a landline from a computer, a computer from a landline, a landline to a landline and also computer to computer like Skype & messenger etc over a broadband connection.
Horses for courses, but with different hardware & software.
happy conversing
