Why can't I write anything on servers (cell phones) in Minecraft?

I regularly play Minecraft on my mobile phone and there also on servers such as Mineplex, lifeboat network etc. However, my chart messages never reach other players. No matter which server I play on, my message is not there. Why?

How do you know they don't arrive

I also call a friend on the phone and he never gets anything or the other answers to my questions / comments.