I would expect this question to be asked more for medical coding jobs. Certification in medical billing is a definite plus, but I haven’t seen that requirement for strictly medical billing jobs. Certification does show you are committed to your profession and meet certain minimum standards. If you are not certified, a good response would be that you are working towards certification - assuming you are. But don't dwell on what credentials you don't have - emphasize the experience and skills you do have.