Qais,
Not sure if the upgrade will help. You need to check what version you are running and the see what version is available on Quintum’s web site. There should be release notes for the new version that will say what bugs were fixed. I have not heard of a problem like yours before, especially with the A400. Have you tried connecting a phone to the line and ringing into it to see if there is a delay in the phone network deliverying the call to the line?