There are two main issues at hand here. One of the issues is when you cannot receive texts or SMS from someone who has sent a text via an iPhone. And the other issue is when you yourself can not send texts or SMS to someone who owns either a Windows, Android, Blackberry phone. Now, as a caveat with regards to the second issue, it is most likely due to the fact that the messages are being sent as iMessages.
How To Fix Apple iPhone X Not Receiving Text Messages
One of the best ways to resolve your iPhone X not receiving text messages issue is by going to the Settings. And then opening Messages, and then Send & Receive. Press on Use your Apple ID for iMessage and sign in with your Apple ID. Check to see if your phone number and Apple ID are listed under You Can Be Reached By iMessage. Now, on your other iOS devices, go back to Settings and then open Messages and then Send & Receive.
If you have lost your iPhone or it is currently not in your possession, it would be correct to assume that you can’t deactivate the iMessage. If this is the case, the best course of action would be to visit the Deregister iMessage page and disable iMessage remotely from there. When you get to the deregister iMessage page, go to the bottom of the page and choose the option “no longer have your iPhone?” Under this option, there is a field where you can input your phone number and choose your region. After that, tap Send code. Input the code in the field “enter confirmation code” and then press submit.
When all of that is said and done, you should be able to start sending and receiving text messages on your Apple iPhone X from other iPhone users.