Phone call is not adding to account

Hi all,

I am having a activities and case subgrid in the Account entity. when i try to create a phone call and convert to case , phone call and case are not setting to that particular account.

What I observed is when I initially create the phone call from account regarding field is prepopulated with Account name and later when I convert to case it is setting up with some phone call subject name.

If the phone call and case are not in that particular account after creating will create aproblem to our business.

Please advise any solutions.

Thanks