a. Printing. On a compositor's frame: either of two shallow trays divided into compartments in order to hold printing type.
If a set did indeed have different sorts for the different types of digits, then the equal-height digits would be in the upper case and the differing-height digits would be in the lower case.
b. Typogr. Each of the two forms, capital or minuscule, in which a letter of the alphabet may be written or printed.
A majuscule form has a generally equal or near-equal size for the different glyphs, with their generally reaching from the base line to the cap-height bar perhaps some curves reaching slightly above or below that to give a better sense of balance (so that they look like they are going from the base line to the cap-height). This is true of the equal-height digits.
A minuscule form varies in height, with some perhaps reaching up to the ascender height (or perhaps just the cap height) and some reaching down to the descender height. This is true of the different-height digits.
We can also consider the uses. When different-height digits are used they are used in runs of text, where one also uses minuscule letters. When using such digits one would generally still use equal-height digits if used with text that was capitalised.
Really, in any use that makes use of these, digits are bicameral and the equal-height digits are correctly called "upper case" or majuscule while the different-height digits are correctly called "lower case" or minuscule. There's no technical argument for calling them anything else.
In a use that doesn't make use of them, digits are unicameral. It still make sense to refer to digits as "upper case" or majuscule, because of the history of how they came into fonts, and also because many fonts do have lower-case digits in them, albeit not easily available to users with most applications.