It's almost always the "hash key" here in Ireland.
For a sample from Ireland, call:
From the U.K.: 0800 890353
From the U.S.: 1-800-562-6262
(Sorry, I don't have the numbers for other countries to hand.)
I remember using some very old telecom service that asked you to "key your ID number and terminate with a hash!" How customer friendliness has improved!
Ah, but they will still tell you your call will be terminated. Just hit 0# each time at the prompts on the above number!
I just checked the BT Direct service, and curiously enough that uses the term hash, even though all the local voice prompts (*55* for reminder call for example) all say to use "square."
BT Direct-
From Ireland: 1800 550144
From U.S.: 1-800-445-5688 (via AT&T)
By the way, you wouldn't want to use BT's chargecard service except in a real emergency. Rates back to Britain are 78 pence per minute from the U.S. and 90 pence per minute from Ireland!