iMessage error: 'you must have a SIM associated with an Apple device' on iPhone

iMessage does not want to connect to my phone number. It just keeps loading and then shows an error. I have tried everything from restarting the phone, restarting network settings, removing/adding phone number, changing region, timezone every trick I found onlline, nothing worked. Does anybody have a solution or what is the problem here?

[Re-Titled by Moderator]

Original Title: you must have a sim associated with an apple device for to be used for imessage and facetime