If you have heard of a digital butt set, it is usually a CO thing for BRI/ISDN or PRI/ISDN trunk testing and last time I looked was around $1,300 bucks for a middle of the road set. If you need to test digital lines, set up a digital set or 2 in your switch room (MDF) or at the IDFs with a jack and jumper wires that you can punch down to see if your set is working from point to point to point. Multi-meter also will do this without the dial tone and display naturally, but you can see your DC voltage and polarity. Meridian 1 sets around -28.5 VDC, if using TSG sets the power pair from Shumway 42 VDC and analog sets approx -49 to -52 VDC, if voice path is established for analog your voltage reading approx -7 VDC. Hope this helps you out a bit or gives you some fresh ideas for your set up testing needs.