Johnaudi wroteDo you have a DSL filter installed? They cost about 5$ just install it on your phone line. It separates the phone signals with the DSL one.
If you have WAN though I'm not sure if DSL filters exist for that.
man the splitter or filter isn't even necessary on the line, it is a simple Low-pass filter which ONLY filters DSL signals from reaching the phone, the phone signals are still connected to the modem. you can confirm that by simply looking at the circuit or plugging your phone in the Modem port on the splitter and you will see that you still can make calls normally.. the sole purpose for them is to remove high frequency squeaky noise to be heard from the phone.
My home setup which is the best, is to find the wire coming to your house from the pole or box whatever(probably two wires with striped ends) cut the the rj45 connector from one end of the modem and solder it directly to the line with no splitters or filters whatsoever.. on the same line solder a filter or a splitter AFTER the modem wire and connect it to the rest of the house. the whole point is to make the modem directly connected to the line comming from outside without any splitter or filter and you should see great stability boost.
Edit: just a simple explanation.
if you look at the image you will simply see that the Modem port is DIRECTLY connected to the input line with no components influencing it, that tell you that DSL doesnt need a splitter to work.
while phone port is connected after several inductors and capacitors which are solely acting as frequency range low pass filter..so also the Phone doesnt need a splitter to work properly its just that it is installed for you to not to hear dsl noises in the phone while "maybe" sacrificing a bit of line quality