What exactly does it mean when an incoming call appears on one’s phone as “restricted number”? Who is likely to have a restricted number? Why would one have a restricted number?
One legitimate example I know of is that women’s shelters often have restricted numbers as an extra layer of security to prevent abusive partners from being able to discover their location. However, pretty much anyone (in N. America anyway) can set up a restriction on having their number shown to the person at the other end, so it is also used by collection agencies, robocallers, telemarketers, scammers, and others for less legitimate purposes.
I am friends with various physicians. When they call me from their mobile, “restricted” is the usual message. So I don’t know which doc friend is calling about getting together for dinner, but I’m pretty sure it’s one of them. 
Intelligence agencies sometimes use them.
The local PD here has a ‘restricted number’. It’s supposed to be used for things like tracking someone down without them knowing it’s the police calling. Of course most people don’t answer calls that come up as restricted, so I’m not sure how much good it does.
I think it probably made more sense when more people had landlines and we’re more likely to answer them regardless of the number (or lack thereof) displayed. Plus, the whole idea was to find out if the person was at home, which isn’t going to work as well when you’re calling a cell phone.
Also, it caused other, unintended, problems. For example, I’m not going to answer a restricted call at 10pm. But then they leave a message, I listen to it, try to call them back but can’t because it’s 10pm and they’re not answering their phones and it turns into a whole big thing.
You don’t even have to set anything up. *67 should block your number from coming up on their caller ID (for that call, you’d have to do it each time if you always wanted it to be blocked).
I have my cellphone set to block caller ID when I make a call. I make 1-3 work related calls each month and the last thing I want is for any of those yahoos to have my number.
Occasionally the person I’m calling has their phone set up to block callers who are blocking their caller ID. For those people I use a GoogleVoice number to call them and I lament they’re blocking my call.
Before the proliferation of cell phones many public pay phones were set up to restrict incoming calls in order to prevent drug dealers from setting up shop on street corners.
Social workers can have these, to shield their personal traceability when they have to talk to dangerous people such as violent prisoners.
Slightly OT, but …
My (Nurse Practitioner) wife’s employer uses Doximity which allows their providers to call from their cell phones but displays the main clinic phone number (a legit “spoof,” so to speak).
AIUI, this resulted in a much higher % of actual connections, since so many people are reluctant to answer a call from any number they don’t recognize, or … no number at all (desperate attempt to be relevant to the OP).
I didn’t have an office phone for several years due to creeping retirement and having only a home office due to COVID. I blocked my number because I was calling patients and students.
I haven’t tested this lately, but you used to be able to dial a vertical service code to unblock your in-place block for just one call. The code was *82. So you could (if this still works, and works with the appropriate carriers) have your doctor friends change your number in their contact list from 234-567-8901 to *82-234-567-8901 and only calls made from their phone to you will show the Caller ID information.
That’s how my iPhone is set up. My friends and those I want my caller ID to show up are in my Contacts List as *82-(aaa)-123-4567.
The mental health agency I go to has its numbers come up private or restricted due to medical privacy concerns. And they ask on intake if they can leave messages on the number you provide them in case you are in an abusive situation.
That will block for a particular call when the default is unblocked. You can also have blocked be the default and as was mentioned upthread, *82 will show the number.
As an aside, I recall that before the days of caller ID, *69 would call back the number who most recently called you. That may still work.