What should I say when I call my future mother-in-law for the first time?

I don't think it is suitable for you to talk to her mother on the phone in this situation, because her mother is biased against you, and telephone communication will be more difficult or impossible. Your telephone conversation can only aggravate her mother's objection.

I suggest that you still have time to go to her house and buy some gifts that her family likes, and then you make a promise, hoping to get her mother's approval.

Try to find time to go to her house, even if you ask for leave. Your calling will only make her family think that you are insincere.