Adding custom field on Activities>Phone Calls

On the custom form field on the phone call, you can select which form to use. 

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You can also customize the form being used on the phone call , by hovering to customize link as shown below. 

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Click New Field 
and set the following

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On the Display tab>Subtab
Select Main
then Save
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The new field for "Cold call"will now appear on the form.