in App Inventor you can use the Phone Call component to dial the phone and make a call
PhoneCall is a non-visible component that makes a phone call to the number specified in the PhoneNumber property, which can be set either in the Designer or Blocks Editor. You can use the MakePhoneCall method to make a phone call programatically from your app.
To directly specify the phone number, set the PhoneNumber property to a Text with the specified digits (for example, "6505551212"). The number can be formatted with hyphens, periods, and parentheses; they are ignored. You can't include spaces in the number.
In case you try do do something else, these Activity Starter links could help: