Hi
A couple of things come to mind
Check that all phones have a filter, this includes caller display boxes
If you have a monitored alarm system that works over the phone line this may not have a filter and could possibly cause problems.
Dont assume that because the filters came with the moden thay are working, unplug all your phone and try the internet, if you still have the problem, change the filter and try again, when you are happy with the connection move the next phone and repeat until all phones are connected
This advise was given to me by my ISP and I found 1 filter to be faulty and an extra one require for the caller display which I had completely forgotten about
Hope this helps
Chris