RE: Can we customize a Quick View form of Phonecall activity?
Hi partner,
Unfortunately, this is impossible.
1. For Phone Call and Task in the social panel, D365 uses a unique Quick View Form called "Quick Form".

Only these two active entities have "Quick Form", other active entities such as Email do not have this form, they use Main Form and use Quick Create Forms on UCI.

2. The "Quick Form" Quick View Form is not customizable, that is, Javascript cannot be set on this form.
In summary, we can't set Javascript on the social panel to set the default value of 'Call With' field of phone call activity.