An echo on a telephone line is normally caused by the 2 to 4 wire converter which also provides local feedback/confindence. It is often a small transformer with multiple taps. An echo canceller can be installed at the exchange but a lot depends on the level and delay.
As for inaudible - it may be that the test is run at a high frequency or is an ultra short pulse that you do not pick up on.
Will it cause problems with xDSL - possibly as the out of phase signal or delayed signal will effectively be "noise" on the line.
What happens to the speed if you just plug in to te test socket with NO phones or devices other than the modem/hub?
Ultimately though, it is a voice line fault and will need to be sorted by that provider.
M H C
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