Details about FAS (False Answer Supervision)

What is FAS?

False Answer Supervision refers to the scenario where the answer signal of a call is modified to charge for non-conversational time. There are three general types of false answer supervision...

  • The carrier returns the answer signal when ringing starts, rather than when the customer answers. This increases the duration of the call, and therefore the cost of the call. A call may even be charged despite being dropped due to no answer.
  • The more clearly fraudulent variant of this scheme involves call diversion. The fraudulent carrier will route the call to a recorded message that plays a ringing tone and then a recording . This is intended to keep the calling customer on the line and paying for the call as long as possible.
  • The call is not terminated when the recipient hangs up, and waits for the caller to disconnect. During the period between the recipient disconnecting and the caller disconnecting, the caller is billed.


How to identify FAS?

  • The billed time of the call is longer than the time for which the called party was on the line.

  • A text-to-speech message has already started playing when the called party picks up the line.

  • The call is billed even if the called party is said to be out of the coverage (for example, for a mobile phone you get an announcement "we are sorry, but the party you are calling is unavailable at the moment").

Why might FAS legitimately occur?

FAS can take place when there is a lack of 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 likely due to incorrect configuration it cannot detect the states of the call, which are returned by the PSTN network (the states are: "called party ringing", "called party connected"). And thus the gateway forces the answer state which normally occurs immediately after the arrival of the call from the VoIP network or after a few seconds. The problem is that the gateway answers the call (which starts the billing) according to its own settings, but not according to the actual state of the call.

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