Welcome to planDisney! I’d be happy to help figure out a way to incorporate Minnie Mouse into your granddaughter’s special day.
The Disney Bedtime Hotline that you found was available for a limited time, but the phone number is no longer active. The only character calls I know of is for guests with an upcoming
Disney Cruise Line voyage. In that case, you can
request a call to hear a pre-recorded message from Disney characters wishing you a magical sailing. The other messages that come to mind are through the
"Hey Disney!" device in
Disney World Resort hotel rooms, where you can activate character interactions like wake-up calls and jokes.
However, I have a few other ideas for how you can give your granddaughter a Minnie Mouse-themed surprise! If you search “Minnie Mouse birthday message” on a video website, like YouTube, there are some clips that you could play. Since they're unofficial videos, I'd recommend watching them fully before choosing one to use.
You could also put together a themed present with a card "signed" by Minnie Mouse. My nephew loves Buzz Lightyear, so one year I bought him a Buzz doll with a balloon tied to his hand, along with a card I signed from Buzz. You could do the same with a Minnie Mouse plush (there are lots of cute options on the
Disney Store website) and if you search “Minnie Mouse signature” online, pictures of her autograph come up. You could use those images to sign her name on your granddaughter’s birthday card.
I hope this gives you some ideas, and I’m wishing your granddaughter the happiest of birthdays!
JoAnna