could be both. short on line, plus so many phones overloading the line
and causing statis. Without the presence of both the short & several
phones it may not show up. So it could still be an external problem.

The multiple phones would have to be off hook in order to load the line
unless you are thinking about the ringer capacitors creating a load for
the AC noise.
--
Tom Horne

Possible, but unlikely... Phone lines typically can power 3-4 "Bells"
or B equivalents. (These are real bells for those old enough to
remember).

The solid state ringers on most of today's phones only take a fraction
of the current needed to ring 1 Bell.

Beachcomber