loud noise in new domophone

Just installed new domophone in school (part of my job)
There is 2 parts of domophone.
One is only microphone/speaker and a button to call. only 2 wires.
Second one is phone. 12V.

Only phone have power supply.
Firs part of domophone takes power together with audio signal through 2 wires.

Cable between those parts is long. Atleast 20 meters. UTP. Unshielded.
with atleast 5 joints. some of them are soldered, others are not.
And through same cable I connected controller (ethernet) and 12V minus through 2 cables. If I connect them then door opens.
TL:DR
8 wires. 4 for ethernet use, 2 for 12V minus of conntroller. 2 for audio signal and power supply to first part of domophone.

The problem is I have loud noise when trying to communicate through it.
At first I used 12V Volt LED DC power supply 2,8A. But friend of mine said that it doesn't fit for audio devices.
If i unplug power supply from outlet, noise dissapears and for few seconds, while power supply still have some power left, I can clearly understand what someone speaking at other side of domophone.

I tried to changing power supply to 12V 1A adapter taken from ethernetswitch.
Sound and noise was even bigger.
Friend of mine suggested to add 1000mF capacitor. But that didn't helped.

Any suggestions?
Sorry for bad english :D