Many people don’t like to start their conversation through voice or video calls they prefer more on chats and email rather than voice calls. These people disable the calls from Facebook Messenger.
People who are getting annoyed with random calls from strangers and friends also like to disable calls on Facebook Messenger. These calls also take more internet data than chats and emails.
Learn how to disable calls on Facebook: https://ityug247.com/how-to-turn-off-call-on-facebook-page-messenger/