Set it up as a cascading RR type of thing. Where in some instances the successful RR does not trigger the trap, while in others it does.
This would allow for you to have a number of options/results available, and the players won't immediately know what the answer is (and you can even include a result where the magical connection created by the spell (Dispel) causes the caster of the Dispel to be affected by the trap, regardless of their range from the trap).