I am getting a busy signal on all incoming call on my iphone 4

This is a carrier problem, and sometimes is caused if you had call forwarding enabled in the past...sometimes it doesn't quite get turned completely off. Regardless, you need to contact your carrier to fix.

Make sure your iphone is not set to " do not disturb".

This can be turnoff in settings.

When activated a crescent moon symbol is present in the status bar at the top of the phone.