Skype is a terrific, free program for voice chat (and text or video chat, for that matter). I've used Skype to stay in touch with my son as he travels overseas in the Navy (Skype-to-Skype calls are free!) and to stay connected with my granddaughter on the other side of the country.
TeachingDegree.org has come up with 50 Awesome Ways to Use Skype in the Classroom. Their list has helpful advice for teachers, parents and students. If you are an educator, be sure to check it out.