No. You keep the number your customers already have. BusyTel gives you a second number, and your line forwards to it only when you decide to set that up.
You pick the area code, later in setup once your card and your own phone are confirmed, and we assign a number in it. That step checks the code has numbers left rather than showing you one. If a particular number matters to you, ask us after you are set up and we will swap it.
Yes: call your new number from your own phone. It answers you the same way it will answer a customer, and the text arrives at the end of it.
Change them from your phone whenever you like. The hours you give here are a starting point, and every answer on this form can be edited later.