You'll need to check to see if your org has Shared Activities enabled and it sounds as though that's the case. When it's been enabled, there's a
TaskWhoIds field that's of Type
junctionIdlist which is a string array of contact or lead IDs related to the task.
This JunctionIdList field is linked to the TaskWhoRelations child relationship. TaskWhoIds is only available when the shared activities setting is enabled. The first contact or lead ID in the list becomes the primary WhoId if you don’t specify a primary WhoId.
So, if you have that enabled, the only way to to create individual Task Records for each contact is by entering the WhoIds into separate Tasks as the RelatedTo or
An option you could try would be to clone them, remove the other contacts, then perform an update/save on each the clones. You could presumably write a trigger to do this automatically for you whenever a new Task is saved where the Activity is a Call.