Sip response code change from 480 to 600

I am trying to apply call limit on my sip trunk. I want to allow only 10 concurrent calls (incoming + outgoing). I have used call-limit in my trunk config and it is working great ( I know call-limit is not recommended) but it is working for me so I am using this. Now the real issue is this, when my voip provider sends the call to mine server and trunk has reached to its maximum limit, my asterisk gives sip response code 480 temporarily unavailable. Due to this call voip provider sends calls to my backup server again. I want to change sip response code from 480 temporarily unavailable to sip response code 600 or 603. Is it possible, if yes then how ? Please help me.