Disable iPhone numeric keyboard auto-toggle
I only have experience with iOS 7 and iOS 8. Sadly I don't have any experience with iOS 6 or lower.

Ok I might have found a solution:

IOS 7, 8 AND 9 beta 1 and 2:

1) Go to settings, then keyboards 

2) Uncheck ALL the options on that page.

3) Test it (in the app where it's affecting you)

OR:

Just uncheck split keyboard and see if that works.