Weird ringiing effect with sipura and sipgate, any ideas?

I have for some time now had a sipgate account, and the majority of the time
it works reasonably well. I do have one problem though... When I call my
sipgate number, and then hang up after a couple of rings, the sipgate line
keeps on ringing for a long time afterwards, sometimes over two minutes?
Any ideas why this happens and if I can alter any config settings to stop it
happening? Its almost like the signal to end the call was never received by
the sipura. Rather irritating when testing the line to make sure its still
working. Any cures, anyone?

John