I'm trying to use a Raspberry Pi Zero to control our apartment unit's buzzer phone (the thing that lets people at the building door in).

There is a momentary push-button that completes a 12VDC circuit, causing the door to be unlocked. I've attached leads so that when I touch them together, it does the same thing as pressing the button.

So far I've tried two approaches to wire this up to a rasp pi. 

The first was with a single NPN transistor controlled by one of the +3.3V GPIO pins. Verified the circuit concept by controlling a simple LED circuit with the GPIO high or low. 
Then I tried it with the buzzer, hooked the +12V from the buzzer to the collector and the 0V to the emitter. This didn't work, and caused some buzzing in the phone.

Next I tried this Opto-isolator breakout <https://www.sparkfun.com/products/9118> in hopes that isolating the two systems would be more straightforward and eliminate the possibility of a ground loop. I also tried this with the LED proof-of-concept circuit. It was noticeably dimmer when going through the opto-isolator vs. not, and it didn't end up working out with the buzzer. I'm thinking it introduces too much resistance.

If you were doing this, what approach would you take? Is there any way I can modify my approaches to make something work?