To activate iMessage and FaceTime using your Phone number
First, check to know whether you have an SMS plan enabled at least for One SMS*** with your carrier. Contact your Carrier (Cellular Service Provider) the agency that issued the SIM to you
If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS*** Only)
If you can't turn on or sign in to iMessage or FaceTime - Apple ...
*** You may not have an SMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS services.