Electronic – Clean/reduce static in electret microphone caused by “dirty” Raspberry Pi pin output voltage

I found the datasheet for the device used on your board here.

First, this is not a buck regulator like the E-Bay seller is advertising it as. It is a low-dropout linear regulator. Luckily, for the input-output combination you're using, that probably does not cause too big a loss of efficiency. Your efficiency will be about 66%, where an actual buck regulator could get 80-90% at the expense of more components on the board (higher cost).

Second, be aware of one other issue that could cause the behavior you saw. This regulator has a minimum load current of 10 mA. If your load (plus the LED onboard the part you bought) draw less than 10 mA, the output voltage could also drift up above the normal operating point.

You mention buzzer phone and that it completes a simple 12VDC circuit, but then also mention causing 'buzzing in the phone'.

Reading between the lines of this I assume that you mean you have an intercom system with a door unlock/release button on the handset and a unit at the door entry ?

The short answer if so is, use a relay it is by far the easiest option.

There are several possible scenarios with an intercom system.

1. The button on the intercom is a dry contact that switches the door lock power directly.

2. Same as above but rather than directly powering the lock the push button is an input to an access control system or similar that controls the lock.

3. The button is actually part of a circuit on the intercom handset, this will then perform some sort of function on the intercom system, depending on the type. Some may then activate a relay on the door station or a similar relay/actuator module somewhere. Some door stations or modules may have powered lock outputs rather than a relay. Typically the intercom may be a bus system with audio/video and potentially data/lock functions on the same wires, some may use several dedicated wires, it varies massively with proprietary yet similar systems used by different manufacturers.

Given you say you caused buzzing by connecting a transistor I suspect that it may be something like 3, where you are affecting your handset's circuit in unexpected ways and if so it may be difficult to determine how to connect a transistor without knowing the circuit design of the handset.