An interesting query

[#1]

I'm now occupying what used to be a council building, and as such the telephone system is odd with numerous extensions around.  There is a British Telecom "Joint" box, which may (or not) be the main telephone box - there are several other candidates that could be the main telephone line, although one can't be used for anything other than phone calls as it is nowhere near a power socket.
Ideally, I would like to get the telephone system running throughout the building, if it can be done as one order.  Failing that, the telephone system for the nearest power socket would suffice.  Until the main socket can be determined, I wont want fibre, and will have to "make do" with just ADSL
The questions are thus :
I presume a residential account will be okay, and wont need a business account.
If the "Joint" box can activate the telephone system throughout the building, I presume there wont be multiple charges or anything like that.

[#2]

A lot will depend on how big the building is. As you say the "Joint Box" may be the main BT wiring into the building, it will then be distributed to various points around the building, via boxes like in your second picture.
The third picture looks more like a date connection point, its not that clear, but may not be in the correct place to patch to the phone system.
Speaking of which, if a largish building, does it have its own PABX, or were there just exchange lines?
Just the start of lots of questions, it may be prudent to ask someone with some telecoms knowledge to help out, and if you are running a business, how critical is your telephony and broadband, will you be able to continue business if its down for a few days?

[#3]

If it is for business use then you should apply for a business account - The support may be better for that.
First thing to try would be to get a corded phone and try it in each socket that will take the plug. See of there is a dialling tone and iff there is
dial 17070 and see what number it comes up with.- Is it the same on all sockets.
The last picture looks like  data sockets which I think is what Mustrum was saying.

[#4]

Its not for business use, it is for personal (residential) consumption.
I do need to see if there is an active line though, so will need to get hold of a corded phone.
There is no PABX system, as far as I can tell - was probably removed a long time ago.

[#5]

Going by the label on the second one, BT Cabling Systems, I think that's probably internal cabling.  The last one looks like a dual RJ45 outlet, part of a structured cabling system.  If there used to be a PBX, as seems likely in a Council building, the lines may have been terminated on a frame or patch panel rather than conventional master sockets.  The lines may have been ISDN in any case, or something weird like private lines from another council building.
I guess if you can't find anything that both physically looks like a BT socket, and gives a dial tone, then you need a new line installed.

[#6]

I did find another BT box yesterday
It would be a shame not to be able to use what is already present - however, I do need to activate the line to see what is working (and where), but that will have to wait unfortunately.
Thanks for your help.