# confused in practical example of shift invariance?

I am reading book of w smith ,"The scientist and engineer guide to digital signal processing" 2 edition I am trying to understand topic of shift invariance from theory given on pg 92

I have attached screen shot and i have indicated the example with in red marks.

I am confused in whether shift in variance ,author is talking about is in regard to which system?telephone line or digital filter? What is our target system in this example that needs to be shift invariant?Is it telephone line or digital filter?

They talk about the shift-invariance (or variance) of the digital filter. In general, the response of a shift-invariant system is independent of a shift of the input signal. The corresponding output is just shifted by the same amount as the input. So if $$y[n]$$ is the response to an input $$x[n]$$, then the response to a shifted input $$x[n-n_0]$$ is $$y[n-n_0]$$. If the independent variable is time, then such systems are also called "time-invariant".