One of the most backwards compatible technology I know of is railway signaling infrastructure - altough it's not a consumer product like a telephone:
There are still fully mechanical control centres where you have to switch the switches by hand and turn a crank (to generate a bit of AC) to signal another control centre the current state of your tracks. Doesn't matter if the next station uses relais-logic or computers, it works.
Speaking of railways, and connectors, the railway couplings in the US are another thing that's withstood the dest of time. The Janney railway coupler is compatible with couplers built 100+ years ago, and of a design dating back to the 1860s, with the refinements pretty much all been making individual components more and more standardized.
There are still fully mechanical control centres where you have to switch the switches by hand and turn a crank (to generate a bit of AC) to signal another control centre the current state of your tracks. Doesn't matter if the next station uses relais-logic or computers, it works.