One option is to use the free SIP standard. It's not difficult to create a free account that you can use to make voice and video calls.
MicroSIP is a very light application that will allow you to make calls using any SIP account. You can create a list of your contacts and personalize the ring tone, among other options.
This is without a doubt an excellent application, due to its minimal impact on your system and its portability, in addition to the fact that you can use it with a number of SIP servers.