Related to my previous post, I have noticed that I am CONSISTENTLY disconnected when I call a particular person and the call is routed into their office voicemail system. When I call the same number from a direct analog phone (not using my Askozia PBX), I can hear clicking on the line produced by the remote pbx as it transfers the call to VM.
May be the click/glitch you hear is detected by the FXO interface like a "far end disconnect" or CPC or a polarity reversal.
Look at the FXO port led on the card, if the light goes out when at the click time you got the cause.
Depending on the country far end disconnection signaling may be available or not (here in Italy we don't have it).
If such a feature is not available in your country nor it's provided from your carrier you can change the FXO setting from the default fxs_ks to fxs_ls; however you can do it a try to check if something change.
[Broadcast message from root@askoziapbx: Crystal ball remote shutdown]