How to enable SMS on my Iphone 12

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 ...

SMS, MMS, WiFi Call, Call Forwarding, Call Waiting, Call Merge, Conference Call, Hotspot, VoiceMail (Visual or Normal) all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.