How can I whitelist specific calls and text messages?

You could set up a Focus with just the people you want to be able to call or text you.

Use Focus on your iPhone or iPad - Apple Support

I have certain Contacts who cannot get a text through. Also, some callers who say they receive a "Caller Not Available" message when I actually *am* available!

How are they getting this message? Are they hearing it when they call? If so, that would indicate a carrier issue as that's not a feature of the iPhone.

Have you checked to make sure that you have not inadvertently blocked these people?

This is exactly what you are looking for...You will get the ring tone only for the people who are on your Contact list. Rest all the calls are silenced.

To turn on Silence Unknown Callers, go to Settings > Phone, then scroll down, tap Silence Unknown Callers, and turn on the feature.

Detect and block spam phone calls - Apple Support (IN)

stillrivercreative wrote:

As mentioned, I already Reset Network Settings via my iPhone, and AT&T claims to have reset network settings at their end. But yes, it may in fact be a carrier issue.

Sorry I missed that you'd already reset. All carriers will try to get you off the line as quickly as possible when you call for customer service. You have to be very persistent.

It looks to me like Apple's idea is that people already in your Contacts are whitelisted by default and that's why no specific whitelist feature exists.

I believe you are correct.