This is configured through 2 new parameters:
ringback-during-collect=yes|no
ringback-file=<wav file path>

You may not want to use this if your E&M lines are connected to traditional phones, otherwise
you will hear ringback tone while pressing digits. This is mostly useful with old switches that do
not provide ringback tone but the user is already done dialing (perhaps the signaling was converted from
ISDN to E&M and the full number was received in a single SETUP message)