Ok I have put a ticket to set the disconnect clear time to be set to 800ms as I have read on multiple places that people had same problem and setting the DCT to 800ms resolved the issue.
http://www.stocksy.co.uk/articles/Networks/how_to_get_disconnect_supervision_on_your_bt_telephone_line/
https://business.forums.bt.com/t5/Phones-phone-lines-and-phone/Line-disconnect-signal/td-p/2020
http://www.voip-info.org/wiki/index.php?page=Asterisk+Disconnect+Supervision
http://www.voip-info.org/wiki/view/UK+Asterisk+Details