This is an easy answer, other than the obvious prepaid cell phones which are a rip off, T-Mobile has a service for bad credit, I know I had it for 2 years. they cap you bill at $150.00 any time you go over that they turn it off. i forget what it is called but if you call them they'll tell ya...
Sprint is also fairly easy to get on with, though they may require a deposit. I have horrible credit and I've been with them for several years, but with a spending limit of $150.00.
Cricket. All you need is an ID and cash.
I found a website that offers free phones with no credit checks and it is a regular monthly plan - not prepaid or cards. I am very happy with the service thus far and i know that they offer international calling. the company is 'approved mobile.'