when i dial *101# through my phone it return my balance status. In my case, asterisk is connected to a lot of phone number where call is placed randomly through one of the free telephone line. It is hard to find out which telephone line is out of balance. Those line which does not have balance are also active but call is rejected. So, I need to catch balance status and inactive all telephone line which are out of balance. And it also notification user for recharge balance.
If you are talking about early media, You must try Progress() before Dial()