Showing the related contacts in Phone call form

Hello Team,

I have a below requirement.

When a CSR is on the Account entity(which is having multiple contacts) and selects to add a phone call from the Subgrid to the Activities>In the phone call form I have to display only the related contacts to the Account where a CSR will have an option to choose from the related contacts.

By default the Call to field is auto populated but that will not be helpful because in most scenarios it will not be the same person who will be calling. 

Any help or guidance to display only the related contacts of the account in phone call form or the similar requirements is highly appreciated.