Phone of the Day: Northern Telecom M5209 Centrex Telephone

On a recent visit to my mechanic’s shop for a car repair, they handed me a box with this brand new Northern telecom M5209 digital centrex telephone.

This telephone will only work on a SL-100, CS2100 or a Genband C20, all too big for a residential home.

These phones were introduced on around November 7, 1988.  See the attached article below from Network World.




The Meridian M5209 telephone set features a Large hinged display. Nine programmable line / feature keys with LCD indicators showing line / feature status. The Meridian M5209 telephone set supports Multiple Appearance Directory Numbers. The Meridian M5209 also has such features as: On-hook dialing allows monitoring of call progress through the phone’s speaker until the other party answers. Feature inuse indicators show status of features such as ring again, call forward and message waiting Hold, release and volume control keys. Compatible with Meridian M536 and M518 add-on modules for line / feature expansion. Calling and called name display show up on the hinged LCD display.

You can find this phone in use still at many government agencies, hospitals, universities, banks, and any large institution that uses Meridian Digital Centrex.   The city of Chicago and the state of Illinois still use these phones exclusively for their own operations.  I’ve also seen them installed at University of Illinois Chicago, Indiana University, Notre Dame University, Northwestern University, State of Indiana Government, Harris Bank Chicago (BMO), University of Michigan, University of Illinois at Champaign Urbana, and believe it or not, at a Pizza Hut in downtown Winnipeg, Manitoba.

These sets are the middle generation of what are technically called “P” phones.  The original sets resembled the Northern Telecom SL-1 wedge sets, while the second generation shown here resemble the Delta M2000 PBX sets, and the third generation sets resemble the Aries II M2000 sets.

The signaling on these phones can be explained by my friend Anthony, who described how these phones use analog signaling to process the telephone calls.

“The centrex phones were designed to be installed on normal CO loops with special line cards at the switch.  They also require AC power supplies with the phones.  Without the AC power supply the phones work, but they do not have enough power to operate the display or the handsfree speaker.

Many places that had large numbers of these phones, used a power supply that went in a closet and powered all of the telephones.

The phones work by hex codes.  For example when hanging up the handset and you put the handset down, the hookswitch generates a hex code that gets sent to the Central Office (CO) switch.  All of the keys and the hookswitch have a hexadecimal code that corresponds to them.  Then the CO sends a signal back that says to close the audio path and turn off the line indicator, and also to update the LCD text. ”

So by what Anthony said above, this explains why even though these phones are analog sets, the phones have instantaneous disconnect when the other party disconnects like a digital telephone or like your cell phone.  Also because of the hex code signaling, the Calling Party Identification also appears immediately on the display, unlike a residential analog Caller ID, which is sent after the first ring.

They definitely do not have the extended feature range of the Meridian 1 M2000/M3900 sets or the personalization of the Norstar sets,  but they definitely allow a clean efficient way to organize multiple lines on one line cord and provide consistent communication sets across a large organization.   Back in the late 1980s, this was a big deal.  Normally users who had multi-line applications had a 1a2 phone with a large 25 pair cable running into the back of the phone.  This new Centrex technology allowed you to have a multi-line phone without the large 25 pair cable on the back of the phone, electronic ringers/speakerphones and more importantly the switching system was located at the phone company office, not at the office of where the phone was used (unless it was in the switching office of the phone company, haha.)

When less than two lines are needed, a user more than likely could use a standard analog line and use an analog Unity Telephone that was introduced around the same time in 1988.  Unity telephones closely resembled the styling of the M5000 Delta sets, but with fewer buttons and plain analog circuitry.  Unity can be used on any analog telephone line whether it was in a business or home.  After Unity was discontinued around 1995, the new M8000/M9000 sets, that can work on any type of line from a PBX, to Centrex to Plain old telephone service,  took the place of the analog Unity.

What is unique, these phones were designed around the mid 1980’s before the release of Norstar.  You can see a red Norstar set in the background, which had just a bit more sleek styling than the M5209 and was introduced in late 1988 but was probably designed later with more ergo aero design, than the M5209.   The Norstar also had a more refined dialpad with recesses in the buttons and cleaner labels to allow for easier dialing than the M5209.

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