I am not able to receive calls from certain people in my contacts

Are those users blocked?

Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch - Apple Support

See the section Manage your blocked phone numbers, contacts, and emails for how to un-block.

Petey682 Said:

"I am not able to receive calls from certain people in my contacts: Some people say they call and the phone doesn’t ring"

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A Few Thoughts:

• Update your iPhone:

Go Here: Update your iPhone - Apple Support. See if you need to install an update - from the iPhone to a Third-Party app.

• Any Security Software Installed?:

Security Software could most retain have the option to block out callers, and filter out you from making calls.

• Something on the Callers End?

They (the callers) may have something on their end. Perhaps, they not dialing your phone number, but rather soon else's