can't select my phone number for iMessage and facetime

i can't select my phone number under the "you can receive imessages to and reply from" at imessage's settings and the "you can be reached by facetime at" at facetime settings. i've tried every tutorial like turn my imessages off and on, change my apple id password, restart my iphone, my ios is the latest version too. i don't know how to activated my phone number, help me please i need to activated it for my school exercise