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If you make the Interest Only payment of $2000, you are paying back the interest portion of the loan. Hence there is no principal reduction. If you pay the 30 year payment of $2200, the first $2000 pays the interest you owe and the extra $200 pays down your principal balance.
If you choose to make the minimum payment $1000, you are not paying the interest that is due the bank that month. You are short in the above example $1000. That $1000 dollars would be added to the balance of your mortgage. (Negative Amortization)
This can be a good loan in some cases, but if you can afford to make an interest only payment each month you would be better off with a straight Interest Only loan.
If the people who are quoting you this rate are not explaining the Neg Am portion of this product, I would stay away from them.