It's not an ordinary "ethernet" (RJ45?) or what I think of as the DSL lead (RJ11?) although it looks very much like that. But the plug is only 7.5 mm wide, rather than 9.5 for the DSL. (That's the dimension spanning the conductors).
However it appears to be the same as the type of lead which goes directly into some phone handsets.
Mr. Google is not being very friendly to me today, it's not *obviously* an RJ series.