J.B. Wood:

Jim, did you finally get some useful info? Assuming that Verizon and
others for traditional analog telephone service are following the old
Western Electric (Ma Bell) standards, then the on-hook DC voltage
between Tip and Ring should be nominally 48 VDC

As far as I know the Tip should be to ground (only at the central office's
side) and the Ring at -48 VDC. This was due to prevent corrosion and ion
migration.

The off-hook voltage should be between 6 and 10 V.