Hi George,
I love hearing from parents who are looking for special ways to surprise their children with the news of a Disney vacation. How fun!
As for your question, I know about a service that should be able to help you pull off a special phone call for your child. It's called
Disney Enchanted Calls and it allows you to choose the character, the type of greeting, and the day/time/number to call. While they don't have an option that specifically mentions a Walt Disney World vacation, they do have one for a "special trip."
Here's a little tip from your buddy Derek:
You may have seen videos of parents surprising their children with the news of a Disney vacation. If it just so happens that you plan to record the surprise you have planned for your family, consider sharing it on the Vine app and entering the DisneySide.com contest that runs through 10/30/13 for a chance to win some awesome prizes!
I hope that helps! Please feel free to write back if you should have any additional questions and I'll be happy to help in any way that I can.
Best regards,
Derek