We have a modem and telephone connected to a mainline(with splitter) using a 1 meter RJ11 cable each. We want to transfer the modem to another offich which is more or less 25 meters away from our mainline(with splitter). Can we extend the length of the cable (RJ11) to 25 meters? or there is another way or correct solution for this problem?
RJ11 phone grade cabling attenuates(slows) DSL significantly. Theres a reason why ISPs send out short cables.
If you want to extend it buy CAT5(e) cable and wire it with RJ11 connectors(on any pair of your choosing, but use the SAME pair(eg blue+blue/white). That'll reduce loss to an almost indeterminable amount. Also wire it neatly so it doesnt get pinched or damaged over time as that'll lead to problems down the road.