Similar thing happened to me a few years back during my college days. If they let you know about this scholarship via email, then email them back. If not, call them before the day of the interview and just try your best to get the message out that essentially you’re grateful, thank you, and that you have a stutter, and for the person to deliver the message to whoever is in charge of the interview if it isn’t the current person on the phone. The day of the interview, I shook everybody’s hand and opened up with a comment “I have an off-record comment but I tried to reach out a few days ago to disclose my speech disability, did you get the message? That’s all I wanted to say. We can proceed as normal.” And turns out they were really understanding.