My Telecom Pioneer aunt gave me a vintage gold colored 2554 set I used on the lines around here many moons ago when I was in college. I took it all apart and reassembled it.
What I noticed compared to a gold colored 554 set was that the network on the (mostly ITT Telecom/Kellogg which I had) 2554 was reduced to a PC card with quick disconnect male terminals soldered onto the board. The (mostly Stromberg-Carlson which I looked at) 554's network used that big box type used on the 500 sets.
From memory, there was a curiosity in the early DTMF cards vintage
2500-type phones. The tone generator ICs originally used required more than just tying the row and column together from the keypad to generate the tone. On the 2554 I had, the Mostek IC required a separate contact closure for the row and/or column to either the positive or negative terminal. I am thinking it was some kind of debounce circuitry to keep the switch from misreading the dialed number. That required extra mechanisms in the keypad which made those extra contacts, rather than these days to use something as simple as cheapie single contact pushbuttons in the keypad. All of that made the tone dial mechanism thicker than a dial rotor. (Don't get me started on those Nortel dial rotors which sounded like agitating a box of rocks every time the dial was used.)That newer network combined with the 2554's single gong ringer probably helped keep its size down. I haven't looked at a new-build Cortelco 2554 to see if the network largely remained the same.
As late as 2002, I know that Alltel New York (now Windstream) around Jamestown charged $1.25 or so extra a month to support tone dialing for residential service. Around 1995 or so, Verizon supplied support for pulse and tone for customers around Buffalo and continues to do so today. Good thing, because there are more than a fair number of folks around here who still have their WE 500 and 554 rotary dial sets grandfathered from then-New York Telephone.
Curtis R. Anderson,
Co-creator of "Gleepy the Hen", still "In Heaven there is no beer / That's why we drink it here ..."