All CO telecom plant is -48V on hook. But some PBX's, knowing that their loop lengths are usually hundreds of feet, not tens of thousands, opt for less, say -24 to -36V.
This is no problem except in one case: line-in-use detection. Fax machines and Zon-Jr type credit card machines often share a line, and expect that -48V to be there and if it is not; wait, as "the line must be in use by another device..."
The Zon, and other devices I know of, have a configuration setting so if you ARE fed by a