What voltage does DSL work at if dailtone is not present?

DSL operates at 0 to 5 VDC. Most ordinary household= lines work off a dedicated line in the Phone Companies Central that handles digital signals only and not the Analog voice ranges.

The speed of DSL is a statement of "Bit Encoding or Bit Stuffing" to process the logical Ones and Zeros.

In order to get a visual of an active DSL line, you would need an O Scope and the freq that it is working at to get your measurements.

A much better tool is a Network Sniffer, which is designed to actually become part of the network and monitor the traffic back and forth on the Network side of the traffic.

Don:

Page 10 here http://datasheets.maxim-ic.com/en/ds/MAX4361-MAX4363.pdf

Would indicate that the levels for DSL are around 28.4 V p-p differential across the line.

Since your typically transformer coupled, loop power nor dial tone appear to be required.