Go to the configuration page of your voip ata and login to admin account then to advanced Setting and next to regional tab………
Fill in the below settings these settings apply for Mtnl/Bsnl India
Acceptance tone : 400@-19;10(1./4./1)
Busy tone : 400@-19;10(0.75/0.75/1)
Conjestion tone : 400@-19;10(0.25/0.25/1)
Dial tone : 400@-19,25@-19;10(*/0/1+2)
Special dial tone : 400@-19;10(2.8/0.2/1)
Holding tone : 400@-19;10(0.25/0.25/1,/0.25/3.25/1)
Intrusion tone : 400@-19;10(0.15/4.85/1)
Refusal tone : 400@-19;10(0.25/0.25/1)
Ringing tone-I (local calls) : 400@-19,25@-19;10(0.4/0.2/1+2,0.4/2.0/1+2)
Ringing tone-II (NSD/ISD calls) : 400@-19,25@-19;20(1.0/2.0/1+2)
Route tone : 400@-19;10(0.1/0.9/1)
Call waiting tone : 400@-19;10(0.2/0.1/1,0.2/7.5/1)
Hope it works!