3-Way Call Detection


I need to find a solution for detecting when the call recipient attempts to initiate a 3-way call, and hopefully prevent it. Please advise.

You need to say how they are initiating it. For a start this may determine whether what you are asking is just difficult, or impossible.

You also need to say why, as there may be a better solution to your problem.

If this is a SIP phone mediated one, and the phone is local to Asterisk, I guess you could monitor for the same device being un-held for two calls at the same time. You might have to allow for transient states whilst they were swtiching between parties but not actually conferencing them.

You can set call limits, but I’m not sure how well they work when one is incoming and the other outgoing, and if you can get it to work, it will prevent enquiries and attended transfers, as well as phone mediated conferencing. You’ll need at least 1.6 to have a chance of the two directions counting against the same limit.

It is for security reasons, the person initiating the call (from the Asterisk system) is only allowed to call certain numbers - and we don’t want the call recipient (not on the Asterisk system) to assist in by-passing the security. So we wanted to know if it is possible to detect this, whether it is a native command in the dial-plan, or a commercial add-on.

Define “not on the system”. If they are really not on the system, how do you even expect Asterisk to be able to know?

You still haven’t properly defined the context (or the technical parameters). What’s to stop the helper taping two handsets back to back? This feels like a classic case of trying to apply a technical solution to a social problem.

It is not a situation where we are trying to apply a technical solution to a social problem. 3-way call barring is supposedly used by correctional facilities throughout the US, who also use Asterisk as a backend. We are simply trying to implement the same, in an environment with the same security requirements. So no, no one will be connecting phones back to back. We simply want to know how to proceed to bar these 3-way calls.

Asterisk 1.6
Audiocodes MPXX ATAs
SIP Trunk / PSTN Connection

Tell us what evidence Asterisk would see of a three way call.

Either you haven’t described the scenario clearly enough, or there is no information that would allow any normal PABX to detect such a call.

If the three way call were initiated by a phone within the system, one could probably modify the source code to inibit refer/replaces, but I see no way that one could detect such an operation performed from outside the system, other than listening for unusual noises, which is probably best done by humans.

Within the system you will still have to ensure that you didn’t use SIP phones with conferencing capabilites.

I ended up finding a third-party / commercial solution which will monitor the voice-payload among other variables which provides events which to act upon. It is from Lattice Incorporated / Science Dynamics. Ended up trolling patent databases to track it down. Thanks.