Unless you're an expatriate, in which case the most useful number might be
08707 940000
, which lets you make a phone call to any of several popular foreign countries, without any secret codes or PIN numbers or cards, for the price of a call at the national rate. Works from cable and mobile phones as well...but doesn't work on weekends.
www.just-dial.com and, no, I don't work for them.
PS to the gazelle: when I try the universal taxi number from all over the Black Country it tells me I have been banned from using the number. Maybe the whole Dudley Metropolitan Borough has been banned but I doubt it...I will keep carrying taxi bizcards in my wallet for now.