Feedback – Why doesn’t it occur?- since Mic and Speaker signal travell same line ate sam

I am NOT talking about feedback using "the air" as medium but directly on the (electrical) line - MY QUESTION IS:
How (or why) doesn't feedack occur, since the microphone and speaker of a door answering system or even on a telefone land line, have the microfone/speaker signal travelling in the same (electrical) line at the same time?

(PLEASE TAKE IN MIND that telefones exist since way before complicated electronics like "real time feedback attenuators" existed - How was this kind of feedback - direclty on the electrical line which the signals travell on -avoided in the early phones since in those days no complicated electronics was used as well)
Although I am an Enginner my area as nothing to do with electronics nor audio - but you may (please) anwer in an "enginner kinda way"
PS - One idea, for instance, could the Mic and Speaker signals be of phase in any sort of way that is really simple and may be a simple rule of physics that we usually don't talk about?

Hi Miguel-Good question!  The old analog telephones used a device that was often referred to as a "network" or a "hybrid".  It was a little box that housed some passive components including a transformer, some resistors,and a couple capacitors.  It took the signal from the microphone, inverted it, and added it to the signal going to the earpiece, essentially subtracting or "canceling" the audio from the microphone.  In fact, it was so good at canceling the signal that it became common practice to add a little of the signal from the microphone back into the earpiece (called "sidetone") so you could tell that the microphone was working while you were talking.  When analog telephones eventually included solid state devices, the passive "network" was replaced with a couple of OP amps.  Those old analog systems depended on the impedance of the phone line being just right or they would do strange things.Newer phones go several steps further, using digital signal processors to not only cancel sound from the microphone, but they also help cancel echo. When hands-free capability became a desired feature, the algorithms in the digital signal processors made it possible.  It amazes me how well these devices can eliminate feedback and even cancel background noise by using multiple microphones and "beam-forming" technology.  The math involved in doing this is way beyond my level of understanding, though. .   .    .    -Rick