If as you say, Samsung acknowledged having a problem with 7100s with high radio on the line, then getting AT&T out there with THEIR chokes, filters or whatever will probably be the only thing that will solve this.
I appreciate you doing the tests. Without the other s/l phones ringing you have proved this problem is not coming in from AT&T ringing all lines at once and strange things do happen.
By busying out the main line, you have proved the multiple ringing problem is not tied to the main line.
When you saw even lines that didn't have a CO line start to ring, it 99.9999% proves you don't have an AT&T fault other than the music.
I often refer people to Mike Sandman and I would trust his radio filters but the problem could be so bad AT&T has to do something else.
I had radio on a CO so bad you could wave the buttset clips by the terminal without touching it and have blaring music and PacBell (at the time) had to come out and literally switch out pairs back to the central office. Just grounding all the other pairs in the terminal outside the office was not enough.
My best guess is that AT&T has to quiet those lines down and a few letters to Samsung about how they need to improve the tolerance to music will at least document in writing they have a problem.
You've done your tests, all that is needed is for AT&T to give you clear lines.
Maybe if you print out all these posts and hand them to the business office they will take pity and get an expert on the case who has successfully dealt with this problem before.
Good luck to you.
