asterisk users - Jul 2005 - All Circuits Busy instead of Busy Signal when calling a busy number using a PRI

When using a PRI on my asterisk system, and when calling a busy number
I often get an "All Circuits Are Busy" message.   I've trouble
shot it
and made absolutely sure all circuits are not busy... but the number
being called is in fact the issue.   How do I get Asterisk to give me
a busy signal instead of "All Circuits Are Busy" when calling a busy
number?  Is this something in the way my phone company is returning
the busy signal to me?