Interesting intercom on ebay:
https://www.ebay.com/itm/Commander-Sytem-DP-1900-unit-tel-key-Key-Unit-Telephone-appears-NEW/114201501374
I have one of these I use in conjunction with an old key power supply, to power up old phones.
You dont actually need a key system to run it. It can be rigged up to a 24 VDC power supply and a ring generator or have it flash a 10 VAC dial light.
Screenshot from above link:
I have used those in the past, but they are very unforgiving if a wiring mistake happens.
I've changed to Valcom units, very ruggid and forgiving and will out last a tone commander by years.
Yes a little unforgiving, but well worth the asking price for someone that has a allot of rotary phones.
I have had blast with mine, just wanted to point out to other collectors that these can be used in an non conventional 'key 1A2' set up with normal phones and a power supply.
(I salvaged mine out of a dr's office that was being remodeled.)