What to say during a mock phone test talking to customers?

Got an essment evening with Pentagon Toyota for Parts Counter Trainee . Heard part of this is talking to mock customers over the phone. Just wondering what the best things to say are when your not sure how to resolve the problem. Had a mock test before and crumbled as I was unsure what to say to the interviewer when they were asking about billing things and I had no clue as they only gave us 2 minutes practice on their system and te information was not on anyway.