Problem:

A customer who recently purchased an ATCOM 4 Port Analog card reported that when a caller calls in the IVR is played successfully. During the IVR the caller disconnects the call but the channel remains occupied and is not freed.

Solution:
Edited the following file: /etc/asterisk/chand_dahdi.conf

Added / un-commented the following lines:
busydetect=yes
busycount=3

Reloaded asterisks and the issue was resolved. By the default the lines are commented as shown below:

;busydetect=yes
;busycount=3

Removing the semi-colon un-comments them and they become part of configuration.