Get all solutions to a query, that has no alternatives when queried

This may seem like an odd question that could simply be answered by the findall/3 predicate. However my problem is a little deeper than that.

So I have a predicate called ran_num/1 that returns one of five random numbers (that is I do not know what the numbers could be but there are only 5 of them).

When I run the predicate it returns this output as an example:

?- ran_num(X).
X = 2
?-

Note that there are no alternative answers, pressing ; will do nothing. Prolog is awaiting another query command.

If I run findall on this the result is:

?- findall(X, ran_num(X), L).
L = [2]
?-

Is there an inbuilt predicate or method I can implement that will get me all the possible numbers that can be generated? So for example I can get a list which is [2,60,349,400,401].

Assume I cannot change the ran_num/1 predicate to give me alternatives.