How to retrieve the phone number stored as work number in Contacts list of Windows mobile? It has several fields like Mobile number,home number,bussiness number... I could able to retrieve these fields as there is direct API in IPoutlook. problem is with Work number.

Most likely the 'work number' for the contact can be obtained by methods 'get_BusinessTelephoneNumber' or 'get_Business2TelephoneNumber' of the object with the 'IContact' interface.
You can try corresponding 'put_***' methods to verify the approach.