I would strongly recommend you contact BT Openreach as they should fix what they broke!
If you don't want to do that, in theory all you need to to is re-connect the disconnected cable. As there is now broadband on the line and a filter face plate now in place of the old NTE5, you're going to have to re-connect the disconnected cable but after the filter face plate and to do that you're going to need one of these:
Simply plug it back in on the phone side of the filter face plate. The issue here is that should anyone unplug it, line 2 on the PBX will no longer work. I have attached another wiring diagram for your reference:
The question is this... Have you located the cable that BT disconnected or are you running a whole new cable to the PBX? If your using the old cable that BT disconnected how many cores does it have and what colors are they? Popping a 430a plug on the end of the cable closest to the BT socket wired as shown in the first diagram above should be all that needs to be done!
If however you're running a new cable from the BT socket to the Panasonic PBX then wire it as above at the BT socket and connect the White and Red to the outer pins 1 & 4 of the RJ11 connector (see 2.2.8 Types of Connectors in the installation manual).
The installation manual can be found here bt the way: http://www.overline.com/assets/Overline ... manual.pdf
I should point out that the manual doesn't say if polarity matters, I myself would wire th RJ11 as follows: white to pin 1 and Red to pin 4. You could use a volt meter on the RJ11 that feeds line 1 in order to discover what polarity is being used on that line.