For my tax office (Cardiff), though I don't know about others, a search using the 3-digit tax office number here:
http://search2.hmrc.gov.uk/kbroker/hmrc/locator/locator.jsp?type=1provides a number 'for calling from abroad', which works. This alternative also gives an honest engaged tone if the current queue is more than x minutes long. The 0845 number, on the other hand, has a particularly annoying reply, "sorry we can't answer your call, please try later", which of course costs me (at least) 8p.