Fault finding on malfunctioning intercom

Scruff

Have a rather old Fermax Bruto 4+N intercom at the gates that is on the blink.
Issues:
When i lift a handset in the house i can hear electrical noise on the line and external noise from the mic at the gate intercom.
Speaker at gate not working.
The buzzer button at the gate is functional but doesnt buzz the handsets.
The gate open button on the handsets doesnt work.

Power is getting to gate intercom from the DIN so what other measurements should i be testing on it and handset to confirm its the gate unit knackered and not a wiring issue?
http://docweb2.fermax.com/docs/manuales/94545%20Kit%20Portero%20Bruto%201%20Linea%20Ref%202235%20V07_05.pdf