OAH was pretty casual when I went through it. It’s set up for individuals to represent themselves regarding ticky-tack violations like this one.
You could try calling 311 again for the number – what did you report, illegal dumping? That said, I suspect the judge will credit your claim to have called 311 even if you don’t have a record of it.
You can always plead for leniency as well – you can admit the violation and seek a reduction in the maximum fine. But do that after presenting the most compelling case of “it wasn’t me” first.