Having discovered, by accident, that I'm entitled to claim Working Tax Credit, I was miffed to know that I could have done so a year ago - no one told me! I was even more annoyed to discover that I have to call a 0845 number to get a claim form. Searching the forum threads, I found reference to a web page giving special numbers for agents to 'jump the queue'. No surprise to find that the page is no longer available!
There is, however, a collection of pages from 'http://www.hmrc.gov.uk/contactus/by-phone.htm' many of which give geo numbers for overseas callers - all starting -44 XXXXX. Just replace the 44 with 0 and you're there - they won't know the difference! Free (or at least no extra cost) if you have an inclusive call plan!