I would start there the Valcom supplys the talk path

Quote
In connection with a telephone system, the unit shall have the ability to provide loop current, the detection of which shall indicate
a request for service (in the loop start mode of operation). Ground start operation shall be available as an optional mode of service
request. In connection with a telephone system page port, a "dry" tip and ring connection with provision for contact closure start shall be
provided. A battery feed switch located on the unit shall enable or disable the loop current option of the tip and ring connections. A
second switch shall select loop start or ground start service request options.

Merritt

Business Telephones & Equipment + Commercial Audio/Video Products
Commercial Communications . . . Turner, Maine
If it was built after 1980 don't expect it to work right.