Backstory: A relative's workplace has recently upgraded their phone system and I got handed an old Panasonic KX-T7636 desk phone that was going to be thrown out.

The desk phone has a POTS jack in addition to the PBX line jack.  It didn't come with a wall wort so my suspicion was that it was powered from the PBX line.  I tried connecting it up to a phone jack and as I suspected, nothing.  

I then decided to tear it down, find the voltage rail(s) and figure out what voltage(s) were needed,  and rig up something to feed  the voltage(s) into those rail(s).  After I cracked it open I studied the PCB carefully for hints.  I found something marked 3.3V on the silkscreen. I probed around a bit with my multimeter and it appeared to be the main and only voltage rail (with the exception of the circuitry for interfacing the PBX and POTS lines.) I also looked up the part numbers of the chips, and the one that I found a datasheet for, showed that it used 3.3V for the supply.

I found an unpopulated SMD capacitor footprint that was connected to the rail and decided to use that as the point to feed the rail.  I macgyvered a power supply using an Ebay adjustable buck converter powered by a 13.8V linear power supply (can you tell that I need a proper bench power supply?) I soldered some speaker wire to the capacitor footprint leads.  Powered everything up. Still no external signs of life.  

At this point I wondered if the phone was trying to communicate with a PBX before it would show anything externally.  Lacking an oscilloscope, I put my multimeter in AC mode and probed the PBX jack. I did get some AC voltages, so my guess appears to be right. The phone isn't going to play nice, until it can connect to the PBX.  I probed around a bit more I found that the lines that fed into the bridge rectifier that feeds the internal power supply only came from the PBX jack.

At this point things have hit a wall. I could try to get more info on the PBX signaling and try to fudge it with a microcontroller, but perhaps that's going down quite a rabbit hole.  I'm not a phone system expert by any stretch of the imagination, just someone that was a bit curious.  Would love to hear from phone nerds if they happen to know anyway to hack it to make it operate as a normal POTS phone, but I imagine it would likely take more effort than it's worth.