I do know that when you get a busy signal, as often as not the signal
> you hear is generated by the switch at your end and the circuit is
> dropped as soon as the remote switch can tell your switch to give you
> a busy. (This is why calls to Europe produce US busy signals rather
> than the local European busy signal.)
And vice versa. First time I called to the U.S. and got a British busy signal it really made me sit up and take notice. Getting an EET (equipment engaged tone) from the U.K. end when all overseas trunks were busy was quite normal in the past, but not a regular busy tone.
The same arrangement appears to be used for spare numbers on some circuits now -- Instead of the North American recording we get dumped to our standard local recording ("The number you have dialed has not been recognized."). It's rather off-putting, because one is never quite sure whether the call actually made it across the Atlantic or whether a BT switch has not been programmed with a new area code and is rejecting the call at this end (which results in the same recording).
Fortunately, this implementation is still fairly unusual, and redialing immediately will often route a different way and give an American recording followed by reorder instead, which is rather more comforting.. It also seems to be very rare to get this digital signaling followed by a U.K. tone/recording when using many of the alternate carriers.
-Paul.