SMPPEX.Session.handle_socket_closed

You're seeing just the callback handle_socket_closed, go back to SMPPEX.Session module for more information.
Link to this callback

handle_socket_closed(state)

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Specs

handle_socket_closed(state()) :: {exit_reason :: term(), state()}

Invoked when the connection is closed by the peer.

The returned value should be {reason, state}. The session stops then with reason.