A little bit all customers need to have a company directory managed by one person (typically a secretary).
Said address book should also be usable by the various mobile devices, with which users should be able to call their contacts using the voice assistants.
The only thing that comes to mind is to implement a SharePoint list which should then be copied / synchronized with the various users. With Power Automate I could set up a flow. The problem is that you work in a user context (that I know) and therefore 600 Flow workflows should be configured for as many users ..
Public folders are not good because there is no integration with mobile Shooting PST imports doesn't seem practical to me