Every apartment will have a management office. This is intended to get someone that would be available at normal business hours to address situations that may need to be tended to. Since every building has a dedicated telephone number, one could call the telephone number, and dial 0 to reach the office. If the manager can not take your call, you would go to voicemail. If the manager is in an appointment, them they should have DND turned on which will send the caller to voicemail automatically.
In most apartments, the office will be in the same space as the public foyer. This is designed to allow visitors in the office without having to go through the building. If this becomes the case for whatever the reason, then they will have to be let in with instructions of how to get to the office,