Saroj,
there could be many reasons for this and without any further troubleshooting it would be difficult to say what is the cause. If, as soon as call hits the gateway you get fast busy, but then the cause code is 16, I find that somewhat strange as typically in this case you may get a cause 41, or 34 or 47.
This would require additional trouble shooting, take logs, look at config and such.
If you would like assistance on this, please contact me at mike_voip@hotmail.com and we can discuss my services.
Mike M
mike_voip@hotmail.com