How to get information about pickup of a dialed subscriber on the FXO gateway?

Question

Asterisk

There is a gateway (Grandstream HT-503). Used as a gateway to access the PSTN.
It is necessary to detect the pickup of the dialed subscriber in order to play a certain record on the line after picking up the handset. Can it be done? I didn't find anything in the settings.
At the moment, the client (softphone) counter starts immediately after dialing, i.e. there is no such detection.
Thank you.

Answer 1

there is such a thing.
called in the common people "polarity reversal".
must be supported by your carrier.
option in the gateway (I don't know if there is or not in this particular Grandstream model):
Enable polarity reversal = YES

Answer 2

Is there an asterisk? In general, in the Dial command, it is possible to play something to the subscriber after he has picked up the phone.