call progress/comfort noise asterisk/a2billing

Due to the long post dial delay I decided to play a ringback to voip customers generated by the Dial application - "r". I believe that the delay comes from the trunk fail-over feature used by some US operators. Asterisk starts playing almost as soon as the DIAL button is pressed on the phone keypad and continues to play until the call is answered (or not - then is disconnected). I am wondering if there is a way so asterisk/a2b can play a kind of a progress tone/comfort noise (short beeps - or something else) only until the terminating operator starts playing the ringback/early media/SIP Progress (which indicates that the called party's phone is ringing). I remember this used to be used some time ago in PSTN analogue lines. The above describes the experience the customer would have. I know that some providers send Progress SIP messages and some of them play early media indicating ringback. Marcin