I see! Then I will start doing some testing and see which one is most suitable for my use-case. Thanks alot for your time!

Ok I see! This is not exactly what I want to do because the app will be used by several people and they are definitely not interested in having lots of unrelevant numbers in their local contact list. If that is the case, then Call Directory App Extension might be the way to go, but I wonder how frequent is it possible to to update the database (i.e., fetch new (or updated contacts) from online db). Like do you know if once per hour is possible? Or does the update occur based on the user's input, for example that the user presses a button in the app and a sync gets performed. Because if one of those ways work then that would be more than enough for my use-case. Thanks in advance!

Just a small update: During the weekend I read a bit on Contact Provider, and it seems like the database of contacts will be stored in the phone's contact list. Is that the case, or can I choose to not show them and instead just use the contact information (from the database) when a (identifiable) number is calling?

Thanks for the reply! I see, then if Contact Provider Extension will let me sync more frequently, then I will go with it. Just before I start looking into building it, do you see any ways where the idea for my app might go wrong with this extension? As I said before, I want people from my business' contact list (that do exist in a database) to be identified if the incoming call's number exists in the database. This is the idea in its most simple form. I might need to ask more in-detail questions later but first I want to hear your thoughts. Again, thanks in advance for your huge support!

Thanks for the reply! I see what you mean, and I might instead go with the Call Directory Extension (briefly explained here: https://developer.apple.com/documentation/callkit/identifying-and-blocking-calls). As far as I understood, I think that you are referring to it as a much better solution to what I want to do, BOTH theoretically and practically. Am I right? Regarding the database, if I can update the pre-loaded on-device database every an hour or so, then it's fine by me. In that case, I might start designing the app with the Call Directory Extension in mind. I want to hear your thoughts! Ps. Call Directory Extension is not restricted to VoIP calls right? Because I want to get Caller ID when an ordinary call (via the default iOS' Phone app) is received.

Thanks for the reply Kevin! I completely understand then. I think either way, I can fit what I need to fit inside the name field. With that in mind, do you think that it is completely feasible that I build a very small app that will implement the Live Caller ID Lookup extension so that I identify caller if he/she is among my business' customers (given that their contact details are already in our internal database) given that I build up the server based on the requirements that you have indicated in the protocol (I mean the privacy-related requirements with the specific endpoints, data format etc.)? By the way, I have looked into using a periodically updated offline database (via Call Directory app Extension) to do what I referred to above (and which was a common solution for lots of developers before Live Caller ID Lookup), but I actually need to have access to an online database since our database changes very very often, and therefore I want it to be always synced with the calling-identification service that I want to build. What do you think about my reasoning? Should I kick-off this project with Live Caller ID Lookup, or do you think I should look at other options? Thanks in advance!

I see! What about how much information that can be inserted into the name field? I guess there is a restriction on the amount of characters that can be put into the name field?

Thanks for the reply! I don't think I was super clear. I actually do not want to link specific users to specific queries. I just want to send the contents of a field in my database to the user who is using the app. Imagine that I have a field in my database that contains the value: "testing Live Caller ID Lookup". I want this string to be showed to the user that is using the app, in addition to the user name and user icon. So that would mean that the screen that the user will be getting, will look like something like the picture below: Is that possible?

Just to be more clear, in the addition of a user's name and icon, I want to add another field!