The memory care facility where my mother lives does not allow a residents to have a phone in her room because they do not want them to make outgoing calls. To reach a resident, a person must call the facility and hopefully reach a staff member who can take a facility phone to the resident. It's often difficult and time-consuming to reach my mother this way. I live hours away, and phone contact is the only way I have of reaching her on a regular basis.
In businesses that have multiple lines (such as a hotel, hospital, or NH, not such as apartment, where each unit has thir own line) the phone company brings lines into the building, they have CPE, customer provided equipment which sets up the "extensions". Depending on the sort of on premises equipment, they may be able to define extensions as call in only. So it can be done, but it is likely out of your hands and it sounds like they prefer not to accomodate. This may be for a good reason, such as preventing calls when the residents are sleeping. Remember, they need to worry about medical patient logistics in the big picture.
Your best way around is a cell phone, but that can be a challenge for the elderly.
When my dad was in the NH we got him a cheap cell phone. He understood that when it rang all he had to do was open it but he was unable to make outgoing calls because the phone confused him. In his NH they didn't have phones in the rooms either and we had to be transferred to the nurses station (once we got through to the receptionist) and the staff member who picked up would have to track my dad down and bring him to the phone, after the phone at the nurses station rang 100 times. It was a mess. The cell phone made things much easier.