Aircall creates automatically a 3-figure extension (e.g. 001) for each teammate you invite on your account.
You can find your own extension in your app Settings > My Numbers. You can find your teammate's extension in your App Contacts > Team, in the details of each teammate.
Because we like simplicity, we attribute automatically extensions to each teammate and they can't modify it. For example, the person who creates your account will have extension 001 and if James Bond is your 7th teammate on Aircall, he will have the extension 007.
[Editor's note: of course we'll make extensions customizable if we get requests for it :-)]
A user extension is valid on any Aircall number (even if this person is not associated to this number), except on IVR numbers.
Callers can press an extension at any moment during a call, except when leaving a voicemail, to be connected to the user.
When you receive a call and you need to transfer the call to another teammate, you can just press the appropriate extension. The caller can do it too on his side if you invite him to do so. That's very useful when you take calls on your mobile (outside of the Aircall app).
When a caller leaves a message after pressing a specific extension, a voicemail is created and assigned automatically to the appropriate user. It appears therefore in his/her activity feed, in the Need to Follow-up view.