u can probbly buy or make from kit a device that goes on each phone soket or appliance so that when one engages the line the others r blocked. this prevents both a telephone cutting the line on the internet connection and others from eavsdropping on multi telephone systems.
If you are using a filter but it's still dropping Broadband, check you have it in the right place. The filter should be in the phone socket in the wall and then the other phone connected into it.
I only know because I got mine round the wrong way once!!
I thought one of the major selling points of broadband was the ability to use a voice phone whilst still connected to the Internet (or am I just lucky?)