Bell spoke as Princess Anna from the Disney film Frozen to an Avery Huffman of California. Her parents played a voice message that was recorded earlier, and posted her reaction on YouTube.
"Hi Avery! This is Princess Anna of Arendelle. I just wanted to call to say hello," Bell says, fully in character, as Avery's face lights up.
Princess Anna says she's heard Anna is a "good girl" and "so brave," and bequeaths her an honorary title of Princess of Arendelle.
Avery Huffman, 4, was diagnosed with an inoperable brain tumour in June. She's just past halfway done her scheduled radiation treatments. Her father Brandon is chronicling his daughter's treatment and the family's journey on a blog.
The video's accumulated more than 144,000 hits since it was uploaded early Friday, and people on Twitter have been sending their messages of support with the hashtag #AveryStrong.