Ring groups issues with PSTN numbers


I have created one ring group having one Asterisk extension and two PSTN mobile phones,

Ring Group 600

100# <<- Works
9799999977# <<-- Works with issues
9797979797# <<-- Ring never comes to this number.

  1. As ring hits to inbound route, it hits to Ring group and first Extension starts ringing

  2. If first extension not answered then Call sent to Dahdi/1 channel for PSTN number but after this PSTN number keeps ringing and ringing this will not go to Next PSTN number, Even if i press busy button on PSTN number.

What i believe that once call sent over Dahdi trunk asterisk has no more control over its signals thats why its keeps on ringing and even on BUSY signal it will not call next PSTN number in above ring group.

Thanks for help in advance.