The RF filters were originaly intended to filter out unwanted electrical inteference which is audible with normal voice telephony. Radio stations, taxi radios, offshore marine transmissions along with whoops,crackles and whistles generated by a local source causing an annoyance when trying to tell the person at the other end of the line, No you're not interested in double glazing. hence they are fitted before the NTE.
Otherwise in the case of DSl, the router is the only appliance connected (normal voice aside) so it could make sense it fiting an RF just before the routers socket.
And as Blacksheep stated, the innards of an RF3 can fit in the back box of an NTE5 (as long as its not a NTE5 elite, you can barely fit anything behind one of those, including a drop wire)