Nortel Meridian 1 / SL-1 / CS1000

Northern Telecom's enterprise digital PBX line, from the 1975 SL-1 to the CS1000.

MakerNorthern Telecom (Nortel)

What callers hearExternal music on a MUS trunk, otherwise a tone or silence

Confidencemedium

Worn loop-cassette (recreation) RECREATION ErrorZap synth: Greensleeves (public domain) on a stretched tape, through a phone line

Listen for

Big organizations, especially in Canada
Admins talk about 'overlays' like LD 14 and LD 16

The SL-1 (1975) was one of the first fully digital PBXs sold to businesses. It grew into the Meridian 1 and then the IP-capable CS1000, which Avaya picked up after Nortel’s 2009 bankruptcy.

What callers heard

Music on hold required an external audio source feeding a music trunk card, configured as a MUS route. There’s no built-in melody. Without that, held callers get a tone or silence, depending on how the system was set up.

Sources

PBX Cookbook: Meridian music
en.wikipedia.org