Text n calls
I can’t text to certain people and even call why?
Try first to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
Then see the following Apple articles:
If you can't send or receive messages on your iPhone, iPad, or iPod touch
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
If you can't make or receive calls on your iPhone
If you can't make or receive calls on your iPhone - Apple Support