False Answer Supervision FAS
From VoIP.ms Wiki
What is FAS ?
FAS is commonly referred to as False Answer Supervision and when this happens the billing for the call starts as soon as the caller dials the number. This is caused by some International providers, usually from countries where there is a lack of regulation, they send a false answer when initiating the call.
How To Identify FAS?
FAS is identified when you are being charged for the call before the called party answers the phone. Many VoIP to PSTN gateways exhibit the problem inadvertently if not correctly set-up.
What Causes The Problem?
FAS normally takes place when there is no synchronization between a VoIP leg and PSTN leg of the call on a VoIP-to-PSTN gateway. When the call reaches the gateway from the VoIP network, the gateway tries to establish a connection with the called number, but due to incorrect configuration it cannot detect the states of the call, which are advertised by the PSTN network (the states are: "called party ringing", "called party connected"). And thus the gateway forces the "CONNECT" state and it normally happens immediately after the arrival of the call from the VoIP network or a few seconds after that. But the main idea is that the gateway connects the calls (which means starts the part of the call, which is billed) according to its own settings, but not according to the actual state of the call.
There are some carriers and some countries who explicitly, on purpose return so called "False Answer Supervision" (known as FAS) signal. Some carriers return this signal immediately after receiving a call while some of them return such a signal after waiting a while (somewhere within 10..20 seconds).
If you encounter these symptoms while placing international calls, as a first diagnostic try placing the call with Premium route (if the issue occurred previously in value, or viceversa) to see if the issue is present in both routes. If yes, then please open a detailed ticket for the VoIP.ms technical support staff, that will review the value/premium route to the number in question, and advise how to proceed, please keep in mind that in some cases of international destinations the FAS issue could be more present and that is unfortunately beyond the control of any VoIP provider.