Telephone line adaptor (-24v to -48V)

What about a 24VDC to 48VDC converter? I found one at model DS25-48 which uses 20~72VDC input & produces 48VDC, 0.5amp (24 watt) and price was $39 each. Check it out.

Post in comp.dcom.telecom.tech. Possibly there are closer vendors than ones like or
--s falke

It all depends upon the signaling details.
If your PBX machine just uses loop signaling for the extension, and the remote telephone gets its talk battery from the PBX you don't need to do anything. If you need 48 volts just to extend the talk/signaling range you just put a 24 supply in series with the PBX battery at the line circuit of the extension. (I have seen this approached used in telco offices where a remote customer ended up with a 70 volt battery!
I strongly suspect that the telephone network doesn't really care whether you have a 24 or a 48 volt battery: when it detects the remote extension is off hook it will send off hook to your machine. And if it detects ringing it will pass that on.