The only time that alphabetical order is the default is when you are sorting items alphabetically.
When it comes to using letters for any sort of input by someone who you expect has some experience in typing, a common keyboard layout makes the most sense.
This will be qwerty for most people, but if you are designing it for any other specific markets for which qwerty is not the standard, I would use that layout (Azerty, qwertz, etc.). This is made easier by the fact that each mobile already has a keyboard preference, so I would suggest using that choice as the basis for you.
If I were to choose only one layout for time or resource reasons, I would stick with qwerty.
Edit: If this were for small children with no typing experience or for whom you were trying to teach the letter order of the alphabet at the same time, I would use an alphabetical listing.