“Do you have DSL? I went round-and-round with AT&T with my problem, including multiple technician visits, until one tech came out and installed some sort of a filter (I think) on the line coming into the house. Problem went away, although I was moody about why the previous techs didn’t do it.”
If you have DSL, other electronic devices on the phone lines will cause you problems. Each of those may require a filter. Electronic devices include most phones except for the old rotary types.
I had the filters they supplied on everything. Like I said, the tech came out and installed something else outside and all was fine.