asterisk users - Feb 2005 - SER vs. Asterisk - call in progress to PSTN

We're having a problem with Asterisk when we try to pass a call off to a 
Lucent PSTN using SIP.  This behavior does not exist with SER:

With Asterisk
An ISDN call is started, at the T1 level we receive ?call proceeding? 
and immediately we receive a ?Call in Progress? just like the far end 
party has answered.

With SER
An ISDN call is started, at the T1 level we receive ?call proceeding? 
which is OK, and when the call fails we get the cause code of why the 
call failed

Any suggestions as to how Asterisk can wait for the PSTN to receive a 
call in progress?

Best regards