There is no compulsion of having a phone number or registering your number with a messenger to use messenger. You simply operate messenger by syncing up your Facebook account with your messenger application.
Follow these simple steps to sync your Facebook account with messenger:
- Log in to Facebook application and minimize it
- Tap on the application store and download messenger
- After it gets installed on your phone open it
- It will ask you to log in to Facebook just tap on it.
Well, it asks you to sync your contact number too but that’s not necessary you can easily skip it and operate messenger without adding up your number.
"Hope it was informative"