The best way is to use vpn. you can easily get an asus rt-n16 router to run dd-wrt firmware and to config one at your home as a server and the parents's as client. this way, they can use pretty much any sip phone without any issue. you can build up a asterisk server on that router too if you add an u***** flash card. With asterisk, you are not depending on any voip provider.
