There is plenty of advice here, and on other threads, about how to contact a jobcentre by changing the last digit(s) of the original number, (which often ended 00). However, to be able to do that you need the original number of course, which is not always available.
However, if you go to the BT website,
http://www.bt.comIn the lower right corner you will see a box headed 'Find a number', click on the tab that says 'business name', then enter jobcentre (NOT jobcentreplus), and the location and click 'search'. In most, if not all cases you will then have the 'starting point' number for each jobcentre. I have tried this with several, including Wrexham, and can confirm it works (at least it does at the time of writing!).
With luck this may save people some time and reduce the need of people to ask for starting point numbers.
Please feel free to disseminate this information in whatever way you think best as it is time to hit back against these rip-off numbers.