I design telephone systems as part of my current job and I think all that happens these days if the loud signal was put in using a telephone set microphone is that the maximum signal gets limited within a safe level.
This is done at various points in the phone network – not just to protect ears but to stop the echo cancellers from going pear shaped when fed with an overloaded signal.
What will blast ears is feeding pure digital noise directly into a circuit. but not only is this complicated to arrange, Ofcom and the Police will take a very dim view of this – it would be viewed as assault, no different from going up to someone and physically boxing their ears.
when this happens by accident (such as in badly equipped call centres where there is a slip on the master timing of a multi-line digital circuit) it can actually lead to a lot of call centre ops going off sick from acoustic shock…
