imessage and facetime problem

Dear all, It is the first time that I use Apple device. Recently, I have bought an iPad 2017 and so far everything was great. However, I have some issues with facetime and iMessage. I cannot choose my phone number as an option in the section of "to be reached". In fact, only my email address (that I created for my apple ID) can be recognized. I signed out from iMessage and facetime, restarted the iPad but still, I do not see the phone number option. I would be grateful if you help me what to do. Note that my iPad is updated to 11.2.6 and does not have cellular and I do not have any other Apple devices other than it.