So I did a little research (I work overnight, it's actually 5am in California) and found that her Halls of Residence are in fact connected to the University system. In other words, she has a 5 digit extension and can call anywhere on campus, or any other 5 digit extension within that network free of charge. The lady at Woodland Court wasn't sure who the service provider was, she alluded to me having to call BT for that information.

As far as the THUS Plc information, how should I use that to experiment with geographical numbers?

So I found out her 5 digit extension, which was 36***. I tried using the last 4 digits with the aforementioned telephone number. It rang twice and then went to an automated message, saying "Lines are temporarily busy, please try again later. O106". Then it repeated the same message about 4 times and disconnected. No dice. Where do we go from here? Is there any significance to the "O106"? That's letter 'O', one, zero, six.

adamgoffin wrote on Sep 30^th, 2008 at 10:04am:

Read the FOI response here.

adamgoffin wrote on Sep 30^th, 2008 at 11:22am:

In which case, I suggest you try 0117 307, but it does say that it is "shuffled", so it may not be 0117 307 + last four digits of ext no. If you get through to one which answers, then it would be useful if you can ascertain whether it's someone in the university accommodation. It's likely that the same telephone system is used (hence the same block of numbers) across different accommodation sites, so it's not forced to be where your sister is.

adamgoffin wrote on Sep 30^th, 2008 at 11:22am:

I suggested 0117 337 because it is a Thus prefix just like the 0870. It does not have to be the case that the underlying geo is the same provider as the non-geo number. In the light of the FOI response, I would go with what it says.

adamgoffin wrote on Sep 30^th, 2008 at 11:43am:

In some cases I believe they may tell you the underlying number.

The number of numbers the university will have depends on how many rooms they have. Clearly it won't be anywhere near 10,000, hence the block of telephone numbers won't be 10,000 long. A guess would be a couple of thousand at the most.

I would try shifting by 1,000 and see what you get, ie, 0117 3070???, 0117 3071??? through to 0117 3079???

Make a note of what you've tried and what you've come across.

Result! I'm getting closer! I got through to a girl at Woodland Court, my sister's Hall of Residence. I dialled 011730711** and she confirmed I was through to the right place. Her 5 digit extension was not related in any way to the landline number though. Now it's just a matter of testing numbers! Do you think all the Woodland Court numbers are likely to start with 01173071***, being that I had some joy with this particular number? The girl was happy to know her landline number too, she says it's going to save her parents a ton of money!

I got it! Created a spreadsheet and spoke to about 60 different people at the Halls of Residence and finally I found someone in her flat. All the different flat numbers were grouped together, however not in numeric order. It looks like all the Woodland Court numbers go between 01173071000 and 01173071100, possibly going even higher than that, but I haven't experimented any further.

What I did find was that the last TWO digits of her 0870 number were identical to the last two digits of her landline equivalent. So for example, this being completely random: 087081487** is a Woodland Court number. The landline equivalent is 011730710** with the last two digits matching. The 5 digit internal extensions, as we thought, are completely randomized. The last digit is the same, but that's it.

I hope this information helps those who have family and loved ones at these Halls. Amazing detective work, thank you all so much for your help, especially Dave!