Unfortunately, with the new IP phone technology... Once the handsets are connected to the router, everything is managed via the router. Date and time are read from the internet network. And phonebook is on the router to configure (if you have access to the router).

Maybe it can still be solved:
-connect the main telephone station to the router via an analogue connection (not wireless).
-and connect the two handsets via radio to the base station (not to the router)
Then you might have a chance (but it's not certain... unfortunately it depends on the router).