In the age of cell phones, that is no longer true. In our vacation house for example, we don't even have a phone service let alone a phone in every room! Everyone in the family has cell phones. Even in our primary house only half the areas have phones whereas all have intercom.

That said, of course there are people with different lifestyles where putting a phone in every room you want intercom functionality might make sense. So at the end of the day, one has to talk to the customer and find out how they like to live rather than forcing a certain systems onto them.

Another issue I ran into which may be solvable was to mimic the intercom functionality fully. In our primary house we had an intercom where we could call the kids to dinner and any of them could speak and say they will be late without doing anything. Can you do that with a phone system? That is, page and then have the mic be on for X number of seconds for anyone to speak?