What do you want to do from MM config in this regard?
If you take the number automatically from a number range then it is an internal number, external number is when the user enters a number manually.
An Idoc does not even care about internal or external number ranges. If you do not enter a number in the Idoc then it takes automatically an internal number. If you have a number in the IDoc then it does usually not check if this is conflicting with internal or external number ranges.
It is possible to pre-assign an internal number in the Idoc retrieving the number via function module NUMBER_GET_NEXT when you fill the Idoc, this gives you a little more control.
But how would you want an external number get into an IDoc other than by a program where you have a dynpro to enter this external number.
If they are not happy, then they must have some idea in mind, how else would they know if they can be happy or disappointed?