A call comes from the pstn to the head office on pbx which is connected to voice gateway then this call is forwarded to
phone in branch office which is connected to voice gateway through a fxs card.
Issue is that as soon as the call is picked up in the branch office a four digit dtmf type tone is heard by the person who picked
the call after which the call goes through normal.
What is the reason for this tone and how to remove it.