I have a WE 505C switchboard setup in my basement. It is fully functioning except for the attendant 'buzzer', which according to the documentation said ringing current was supplied from the CO. I lieu of ordering up some trunks I decided to try and build my own. Using information from this forum, I purchased a PowerDsine unit, then paired it a cycle timer relay from Amazon and some batteries. Tucked everything inside a WE 105A box.
The unit operates when the the switchboard buzzer relay closes. That triggers the cycle timer to close its relay for 1 second (ring) then open for 2 seconds (pause). When closed 12V DC is sent to the PowerDsine module to output the ringing current. The ring cycle continues until the attendant acknowledges the signal by operating the proper switch or an off hook extension hangs up.
I attached a short video and a pic of my concoction hoping someone can find it useful.
I should add that I currently have connected to a extension ringer. I find the switchboard buzzer unsatisfying ;)