How to sync contacts from server to mobile devices (android, ios)?

Question

Android

Hello!
Interesting problem, how would you solve it?
We have a list of contacts in a .csv file or in Active Directory.
You need to sync these contacts to 200-250 phones (Android and ios). and so that the synchronization is in one direction (so that the user cannot make changes to the database on the server). There is an idea to do this through the google API, but this is only for android.
At the same time, without exposing Active Directory directly to the Internet.

Share ideas.