Contact accounting first (562-6230, ask for travel group). If this was not supposed to be your charge, they will be able to advise you where the charge originated (BTA versus UPT versus Car Central Bill) and who to contact. When you call next, have the IDT number, the amount charged, and the person listed as the traveler ready to give. You may be asked for further information as well. That person, if they reconciled the charge originally, will need to find the correct Project ID and send it to accounting to have the ledger corrected.
If this was simply started with the wrong Project ID but the charge is yours, contact accounting (562-6230, ask for travel group) and they will advise you what is needed for the IDT correction. The request is usually initiated by a written memo from your department with all pertinent information included.