You first need to use 50 karma points to start initiating a chat with someone and then you can chat with them (it's easy to earn points; click here to find out how). You can also buy points at anytime.
Whenever you are in a chat with someone, click on the $ to select which options and contact methods you would like to use with that other users (for example calls or video calls). Some people prefer to chat only while other are fine with calls and video calls.
Remember that if you enable calls with a user and that user doesn't enable them for you, you will only be able to receive those calls but not place them. This is valid for all the different options found in the $ menu of your chats.