What does the Flash key do in SIP?

Bote

From: Alex Pierry
Sent: Tuesday, 29 March, 2016 08:25
To: freeswitch-users@lists.freeswitch.org
Subject: [Freeswitch-users] Attended Transfer with one-way audio

Hi All,

I'm facing a strange behaviour in FS after a Attended Transfer.

For the A leg listen the C leg, C needs to press some DTMF key. Otherwise only C leg can listen A leg.

…

An important point is that all transfers are made pressing "Flash" key in SIP Phone not by att_xfer in Dialplan.

So, any clue to start?

Thank you,

Alex.