Hello, Anbu.
Let me jump into your discussion and suggest to look at the following point.
The actual code which prevents in IC (at least in CRM 7.0 and subsequent) to leave mandatory fields of Interaction Record empty is in CL_ICCMP_BT_BTSHEADER_IMPL (component ICCMP_BTSHEAD view BTSHeader) in method IF_CRM_IC_PREPARE_FOR_CALLBACK~EXECUTE. If the veto is raised there then the saving process is stopped. Also it's possible to register own listeners and implement IF_CRM_IC_PREPARE_FOR_CALLBACK according to your needs but if additional requirements exist.
Please also refer my answer in some other discussion: Re: End Button Help on Interaction Record - IC
So I'd suggest to check if there is the similar logic in 6.0 version as far as I do not have an access to such system.
If the method in place then the next step is to debug it and see why it doesn't work.
Hope this will help you.