I have learned (the hard way) that you can connect filters together on the adsl side, the RJ11 side, (I hope that is the right way to decribe it) so, for instance, a filter plugged in downstairs into the master socket will allow a phone to be added there and then an extension plugged into the router (RJ11) side taken off to a remote phone point where a second filter can be used to support a further phone and the router itself.
HOWEVER a filter plugged into the phone socket of another filter will allow ONLY phones to work and not a modem since the ADSL signal (I assume) is what is filtered out.
IN short filters can work in series on the ADSL side BUT NOT usefully the phone side. Might help somebody to create a tidier solution.
BTW if your extension to the remote socket terminates with a standard phone plug then simply by an adaptor to enable to use of the adsl side with an RJ11 plug - this link might be helpful
http://www.solwise.co.uk/telesun.htm