Have a old partner II with incoming call ringing being heard on existing calls . Sounds like motorboat thru receiver and extention speaker. Verizon copper co's but they supposedly checked ok per Verizon tech. 2 cabinets with 4 206 mods and 3 400 modules. Custom line appearances where they start with line 5 through 20. 1 to 4 are not used. External vm with 15 phones. Any ideas? I'm being subbed out to check on this problem.