can i use my own modem with flex tv?

Larry in Tech support just told me that I can't use my flex box with my own modem and was very rude to me and didn't want to help me set up.  I asked to talk to his supervisor and he choice not to transfer me to one or give me his employee ID so that I have a record of who he is.  Yet I am seeing that you can use the box with ones own modem.