Display "harvesting" 1. - Power Up

As I have nothing more to do with the logic analyzer now (there is national holiday here in Hungary and I'll have other things to do on the next week, I've no chance to get a new quieter fan and work with it till 2nd November), I picked something waiting to me from the drawer.

I've around fifty of this. I've no PBX for this anymore, so it is useless except for some parts "harvesting".
First of all I disassembled one, and realized, that the only part can be useful is the display unit. I need to reverse engineer a little bit to find out, how can I drive it.
I was looking around but nothing found on the internet. No information, no service manual, no schematic design. In addition the display driver chip is the regular silicon mounted on the board type:

(how I hate these)
So, the first thing, to power it up somehow, because without the PBX, I not even know what kind of power source is needed. It was obvious for the first look that the main board contains a switching regulator:

I looked to the regulator IC, it wasn't familiar to me:

But I assumed that -5 mark means 5V. If I connect a little bit more than 5V to the +- pins of the rectifier bridge:

It will bring the unit alive. And around 8V:

The unit started.
To be continued...