Hi Amanda Liang, thank you for your post. Kindly try updating the firmware by following the instructions from this article: How to upgrade the firmware for Huion product. Please take note that you need to use a Windows computer to update your firmware. After the update, try your tablet on your phone again.
If updating the firmware does not resolve your issue, please manually activate your tablet's phone mode by pressing button 1 for at least 3 seconds.
HUION Customer Support