Define this relationship:
Table A::IDNumber = Table B::IdNumberList
From a layout based on Table A, a reference to Table B::IDNumberList will be empty unless IDNumber is one of the return separated values in IDNumberList. This works because a Value in IDNumber will match to any number in IDNumberList--an Or type of relationship instead of And.
Note: this produces a many to many relationship without using a join table. While this works and can be very useful in many situations, often using a join table to list the values instead of NumberList is a much more flexible option.