Or how many other things are hanging off the same line...fax, etc.

"Stuff" handing off the line within the house just isn't much of a problem
anymore. The "ringer equivalent" is often less that 0.1.

The "outside" plant of the telephone company often includes long runs of the
same pairs going off in different directions. You have NO control over
this but it is MUCH more likely to affect your service than answering
machines, extra computers, extensions, portable phones, or whatever else you
put on the line.

Or the quality of the connections at the external junction box...those
can oxidize very badly and cleaning them up can help.

Maybe.

"Sometimes" when you have several runs from the "network interface" box one
might go bad. But usually this is a case of working or not working. The
reason is that 50' of less that perfect cable inside the house is as nothing
compared to miles of wiring outside your house (including wiring that just
runs off to nowhere.)