From the ACS R6 manual: The PARTNER system will forward only calls on lines that have reliable disconnect. For these
lines, Hold Disconnect Time (#203) must be set to a value other than 00 (No Detection).
Perhaps your phone lines are not actually providing reliable disconnect, (and the ACS thinks they are), or the hold disconnect time is set incorrectly.
Check for reliable disconnect on a standard CO line by calling into your system, and having someone place the call on hold. Then, hang up. The ACS should recognize that the call was abandoned. If the call stays on hold, then your settings are bad, or the CO isn't providing the disconnect signal.