The 3CX team should build a SIP trunk profile for Airtel to work. As far as I know, currently they don't have any SIP trunk providers based out of India. You can try updating Registrar/Server/Gateway Hostname or IP and Alternative Proxy from any existing SIP trunk with Airtel IP addresses, but I doubt it will work. However, it's worth giving it a try. You might need to play with Inbound/Outbound parameters.