Why can’t I use my phone number to set up my iMessage

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 ...

If the above process does not work then simply disable WiFi and enable iMessage over 4G

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Never post a screenshot with any personal credentials, such as email id, apple id, IMEI number, Serial Number, Phone number, Order ID, if you are sharing a map ensure your location is not identifiable. Etc.