Calls go straight to voicemail after one ring or no rings
Text messages stay as green bubbles if they were iMessage before
iMessage texts show as delivered less often or not at all
FaceTime calls do not ring through
You cannot see their updated profile photo or status changes in some cases
Their last seen or online activity is unavailable if you used shared apps
Repeated calls and messages still do not get a response
Try calling from another number and see if it rings normally
Try sending a message from another iPhone and see if it delivers
Check whether the number is still active by contacting them another way