I use "obselete" Australian "800" series (electromechanical) plastic telephones (c1960's) as intercom units.
See circuit diagram.
Configure gravity switch (GS....) wiring as shown.
A and B represent the series connected mic and earpiece in the handset.
System runs on a 3 wire (or 2 wire plus earth return) configuration
An external trembler bell is used for alterting purposes.
Internal bell requires +/- 75 volt to 'tremble' therefore will not work in this design.
Further information "visit this web-page"
Otherwise I would have used it instead of needing an external bell (12 - 20V)
Reverse biassed shunt diode supresses back EMF from bell solenoid coil.
Upon lifting handset; then rotating the dial (which places a short circuit across L1 & L2) will cause the far station to 'ring'
Simply speak into handset when answered.
Uses + and - 19volt supplies.
19V will ring the bell.
When both handsets "off-hook" 38v total is the supply voltage.
More than one handset can be used at one station; an diode 'OR' gate is used to allow "ringing" voltage to the common annunicatior bell.
So you can display these old phones and also utilize them as an intercom!