Text messages.

Hello,

1. In the Settings app, tap Cellular. Make sure that your phone line is turned on. If you use multiple SIMs, make sure that the phone number you want to use is selected and turned on.
2. In the Settings app, tap Messages. Turn iMessage off and then back on.
3. Tap Send & Receive.
4. Tap the phone number that you want to use with Messages.

If you have issues when you try to receive a FaceTime call after you set up a new device, you can also update your FaceTime settings.

If you can't send or receive messages on your iPhone or iPad - Apple Support

Are these SMS messages (green) or iMessages (blue)? If they are SMS messages, contact your carrier and ask them to look at your account. Also, have the people with whom you're having this issue contact their carrier(s).

Have you checked to make sure that you don't have these people blocked?