I doubt it's any different from the standard red/green but nothing
prevents you from saving the plug you just cut off and looking at its
wires (plug is transparent - everything should be visible). Whatever are
the colors of the wires connected to center pins - those are the only ones
you should worry about.
It would not be a good idea to not have a modular plug when you use an
adapter such as DSL filter on a phone line. It is exactly why the
connection is not permanent. What if you need to connect a DSL modem to
that outlet? then you'd want to filer out which is a pain if it's
hardwired. That said though, if you are certain you are not going to want
to filter removed from the outlet, I think the better way would be to use
a phone faceplate with filter inside. Basically, the thing replaces the
faceplate and the jack with an all-in-one assembly and the one I have
laying around in a box of old parts somewhere even has two prongs to hang
the phone on if it's a wall-mount location.
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