Legal Question in Employment Law in California

Yes.

The issue is whether you are entitled to any pay for that shift in not brought in, on the theory of 'on call' time that must be paid.

You first would have to assume that the city agency is violating the rules in this, even though their policies are extensively reviewed by legal counsel.

You can contact the Labor Commissioner to present an unpaid compensation claim with your specific details for them to review, and have them give you an opinion. If they take your claim and open a file, then you might consider hiring an attorney to represent you, if the amount in question is enough to justify incurring fees. You could be entitled to the amount owed, penalties, interest, and your attorney fees if you win. If serious about hiring counsel, feel free to contact me.