Hi Lal,

There is no simple way to do this, but here are two methods that will "Lock down" the IP Phone.Have a look;

The Cisco CallManager Extension Mobility feature allows users to configure any Cisco IP Phone 7940 or Cisco IP Phone 7960 as their own, on a temporary basis, by logging in to that phone. After a user logs in, the phone adopts the user individual user default device profile information, including line numbers,Long distance access, speed dials, services links, and other user-specific properties of a phone. For example, when user A occupies a desk and logs in to the phone, that user's directory number(s), services, speed dials, and other properties appear on that phone; but when user B uses the same desk at a different time, user B's information appears. The Cisco CallManager Extension Mobility feature dynamically configures a phone according to the current user.

From this doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803edae9.html

Cisco CallManager Extension Mobility

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803f3a8d.html#wp51829

Or you could use Forced Authorization Codes (FAC);

The Forced Authorization Codes (FAC) feature, which allows you to regulate the types of calls that certain users can place, forces the user to enter a valid authorization code on the phone before the call completes.

From this doc;

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803ed710.html

Configuring Forced Authorization Codes

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803f3af7.html#wp1078125

Hope this helps!

Rob