they probably would work on a USA telephone set; but the audio levels and other electrical characteristics of a telephone line are all different between telephone networks in different countries (and sometimes even in different regions of the same country)
The exact tech papers are complex and run to about 960+ pages in the most recent ETSI standard; Americans often ignore the lot and then wonder why folk don’t buy their telecoms equipment and/or complain about the flaws in their open source software.
USA phones (and cheap China ones based on their standards that sometimes turn up at market stalls/pound shops) are often “quiet” if connected to UK/EU phone lines from Openreach.
worse still they can draw way too much electric from the circuit which can cause NTL/VM linecards to auto disconnect the line and in some cases even overheat and blow up the SLIC chip in a cheap VOIP adapter (a real nuisance as the whole thing must be scrapped and reprovided; although the device is cheap its a half days work setting the thing to use British/European signalling, tones and dial plan).
TBH its likely illegal in many EU countries to connect this device into the phone network as the mains power unit doesn’t appear to comply to Euro standards and/local Communicatoins Ministry regs and could leak strong voltage to the telephone line if it went defective (which is just as hazardous to the spies as anyone else involved!)
