Most people start with the phone because missed calls are the most obvious loss. What usually happens next is that the same problem turns up somewhere else, and the fix is already on the platform.
Phone first, then whichever channel is leaking most. There is no fixed path and no bundle you have to buy.
No. They share knowledge, not capacity.
It depends on the plan, which is on the pricing page. We would rather you read the current number there than a stale one here.