The 1A2 wasn’t one product. It was a Bell System standard for multi-line business phones: the desk sets with a row of clear buttons along the bottom that lit up when a line was in use, all run by racks of relays called the Key Service Unit.
What callers heard
Base 1A2 equipment had no audio path to a held line. Hold was silence. Music needed a separate add-on card wired into the KSU, and whatever the installer connected to it played through an isolation transformer. Usually that was a radio.
Confidence is low because we’re working from key-system references and collector sites, not a Western Electric manual page. The silence part is well supported. The specific card models are from forum lore.
It also predates the whole idea. Alfred Levy’s patent for a “telephone hold program system” was filed in 1962 and granted April 12, 1966.