What FREE options do I have to communicate with another UBUNTU user most like an actual phone w/o add. hardware ???

My mother has UBUNTU and we would like to use it to talk to one another as closely to a telephone as possible but for free. I know there is a ekigia and a gaim, I dont know if they are free or how they work.but then how does skype come into play ? I have very limited space in my hard disc,so I didnt download skype not knowing . We are both new to UBUNTU. I am willing to learn how to do the things,she isn't so good with computers. Cant I go into her system and set it all up for her ? If so, any help or instructions to do this also would be great.