Automatically Source Standard Mobile field from entity record to Phone Call page

Recommendation:

1. Navigate to Customization > Lists, Records, & Fields > CRM Field.

• Enter Label (E.g. Custom Mobile)
• Type = Phone Number
• Store Value = False
• Applies To > Phone Call = True
• Display > Subtab = Main
• Sourcing & Filtering > Source List = Company
• Sourcing & Filtering > Source From = Mobile Phone
• Save & Apply To Form
• Select Custom Phone Call Form
• Click Save

2. Access any Lead Record with Type = Individual.

Note: Make certain Mobile Number has value stored.

3. Click Communication tab > Activities subtab.

• Click New Phone Call button 

Note: Notice that New Phone Call popup page display custom field "Custom Mobile" with mobile number from the Lead record.