"install filters on the phone jacks"
I can't count how many times I've found missing DSL filters. For now you need to make sure you followed their install notes and have the filters on all devices save the DSL modem.
Also you can test which phones interfere and which don't. NO ONE will tell you which should and shouldn't with complete certainty.
my wireless internet works fine. my phones (cordless 900 MHz) work fine. when someone is on the phone, it kills my internet connection. my ISP tole me 1) phone company problem (ha ha). 2) install filters on the phone jacks (no help). 3) connect directly to my computer & not use my wireless router (same problem). 4) its my 2.4 cordless phones & I need 5.8 (they are 900MHz not 2.4). I have disconnected the phone line to my printer. no help. is there a solution other than throwing the cordless phones in the trash? we live in the country and nothing in the area that would be adding to the problem I can think of.