You can find your REFERENCE ID in your statement, see the sample below
SECOND OPTION, MAKING A PHONE CALL
If You Choose To Pay A Bill Using Your Telephone:
Please clearly leave your name (or the name of the person who services we provided) Make sure to include: (1) Date of Birth (2) First Name (3) Last Name (4) Date of Service(s) [include one or all days in question(s)]