Completed calls still being shown as Acrive

We have just installed Trixbox CE 2.8. We have 5 extensions using Aastra 53i's and 9133's. We have 2 trunks from sipgate.
The problem we have is that, occasionally, and I stress occasionally, a completed call continues to be shown as active even though the user has hung up the phone. The same extension can be used to make further calls which also may or may not be shown as active when the user hangs up. This problen has so far been restricted to outbound calls and the odd internal call.

I have modified the 'Asterisk Dial Command Options' and 'Asterisk Outbound Dial Command Options' to automatically drop the line after 57+ minutes but this is not the solution to my problem merely a work-around. On examining my sipgate account I see that sipgate correctly bills me for the actual call time, it's just the Trixbox server that continues to keep a channel active.

Any ideas?