Hello,
I can find out the actual reason of asterisk to stuck on 120 calls.
I can send 120 simultaneous calls to your server to see asterisk behavior.
During this time, I will generate some logs to find the real cause of this issue then
I would suggest you a solution to fix it.
May I know which Asterisk version you are using?
Are you having Asterisk, MySQL and A2Billing in same machine?
Can I have the hardware specs of your server?
Regards,
Aneel