My friend Trent sent me a pic of one of his office phones. The M5209 is a Meridian Digital Centrex multi line telephone that can be used on the legacy Nortel DMS or SL100 platform. Many North American universities, government offices, banking institutions, television stations, and hospitals used these phones during the 1990s.
The M5209 was modeled after the SL-1 M2000 Delta sets. An analog Unity version was also available wth the same styling. The M5209 sets were redesigned in 1995 to match the M2008 PBX set and they were renamed the M5208.
The M5208 sets are still manufactured today by Mitel, who acquired Aastra, who acquired the MDC telephones from Nortel in 1999. The M5209s are still widely available on the refurbished grey market.