I'm not excatly clear about your reference to the extension plug being RJ14. Do you mean the plug which fits into the master jack to feed the splitter and extension? That's not an RJ-anything configuration.
The standard British BT431 plug is 4-conductor with the line on the OUTER pair of contacts. One of the inner contacts is also the bell feed (capacitor in the jack), which may or may not be used by the telephone depending upon its design/origin.
Are you sure you have the wiring connected correctly at the extension jack? Standard coding for a basic line is blue/white on 2, orange/white on 3, white/blue on 5. The DSL modem could be getting enough signal to operate by capacitive coupling in the cable run, or if the connections are incorrect you might even have it connected to the line via the 1.8uF ringing capacitor in the master jack.
Paul