imessage alert popping frequently

imessage alert popping frequently saying my carrier may charge when using iMessage..i already turned off iMessage but keeps popping the alert when turning ON data or wifi and even restarting said iPhone.this happened once in iOS 10.1 and when turned off,never come back.i am annoyed by the iMessage alert everytime i unlock my device..

i just recently updated to iOS 10.2.1(iPhone 5s)

thanks,

Tiago