BT Inspiration Asterisk integration hangup on extension

I’m currently in the process of migrating a BT Inspiration system onto Asterisk. As an intermediate solution we are connecting a TDM400p card to an analog extension of the Inspiration. This all seems to be working okay except I cannot get asterisk to detect a hangup. If I connect the asterisk box to a standard BT analog line, Asterisk detects the hangup without problem.

The only way that Asterisk can detect a hangup when connected to an Inspiration extension is by using the callprogress option. Unfortunately when this option is enabled if I pick up the BT inspiration extension using a sip phone through asterisk, the asterisk box thinks the extension is busy.