Nowadays WhatsApp is the most using mobile number based messenger among the world. Using a low bandwidth internet connection you can chat with anyone who has on WhatsApp.
There should be a demerit for any technologies. Misusing is the main demerit or problem of any technology. Here WhatsApp is also misused by harassing others. So WhatsApp allows users to block anyone. If you block someone on WhatsApp, then he/she can’t chat with you. Like this if someone blocked you, then you can’t chat/call with him.