IOS devices are a little easier to determine than other providers. There are so many devices out there by Samsung, Sony, LG, Motorola that its tough to determine all phones that your consumer may have. Best bet would be to test the two most popular devices from each provider.
Unlike desktop computers, the physical user interfaces of mobile devices vary tremendously. They vary by screen resolutions, screen sizes, and button placements. They also vary by Operating System and versions. You could buy either a representative sample of devices of you could use a simulator.