It's pretty standard practice to use dry contacts for interfacing between the elevator controller(s) and third-party hardware such as a building security system. This is done to specifically prevent problems such as the one you appear to have experienced where different voltage references for the "common" or "return" / exist. As a result, what one system sees as "ON" (which might be +24 to +30 VDC), the other might not see as the same voltage level.
Sometimes systems will use opto-isolators on the inputs and outputs to solve this problem, however that's not what you'll find on an Otis RS-5 board.
