With out sounding funny, did you check that the lead is definitely terminated correctly with 6 pin RJ12 jacks and that pin 1 on one end lines up with pin 1 on the other end?
The fact that the signal gets through with barely inserted connections made at the DCS and the UP suggests to me that it is not a UP or DCS problem rather a cable / wiring / earthing / ? problem.
The fact that the signal gets through with barely inserted connections made at the DCS and the UP suggests to me that it is not a UP or DCS problem rather a cable / wiring / earthing / ? problem.