Step 1: To have a video or normal call, the prerequisite is that your phone must already install messenger app. If not, you need to download and install it with the following link.:https://itunes.apple.com/us/app/messenger/id454638411?mt=8&ign-mpt=uo%3D2
Step 2: Choose People, and then choose Active. There will be a list of online friends and you just need to choose the one that you want to have a call.
Step 3: To call your friend, just click on one of the two symbols as below: