Giving your bank details etc. You should have been given a copy of that. Check it out.
You may have been given the wrong Form, which was wrong on their part, but you must bare a small portion of the blame for not checking what you were signing.
If the Form is the one for payment before the 6 months is up, then a polite call telling them of their error will, almost certainly, get an apology.
You will find that most people react better to a polite approach, and tend to become defencive if shouted at or called incompetent when, in all probability, a mistake has been made.
Good luck ~ I would suggest that a quiet, pleasant, approach will get you further than a tirade about their incompetence.