Q: How to enable other people to send imessages to my phone number?

If it is still unchecked after you turned it off and then back on you need to contact your carrier to make sure that silent message to Apple servers is not blocked. It will not be activated unless that message goes through.

Anybody can attach apple id, but in order to attach phone number Apple server is sending/receiving one text message. You do not hear it -"silent". They do not get it - "not activated". There is no way around that part of activation = your phone has to be able to send/receive that message.

http://www.igeeksblog.com/fix-imessage-waiting-for-activation/