I am going to show you, how you can install telephony server Asterisk on a small computer RaspberryPi in order to be able to make calls from your computer or smartphone to ordinary phone numbers. So basically, you will end up literally with your own PBX in your pocket. Actually no, because RaspberryPi needs to be connected to power supply and network, and you would look weird with cables going into your pocket, but you get the idea.