Harada Keigo explains how she got the idea for Phone to Heaven:

“I was very close to my grandfather, but as we lived far from each other in Japan, most of the time my relationship with him was through the telephone.

After moving to England at the age of 13, I called him less and less. After he died, I found out that he was always waiting for my call and kept asking if it was me when the phone rang.”

Phone to heaven is about trying to contact the dead. The bubble maker is activated by a voice sensor in the telephone: The faster you speak, the smaller the bubbles are. The slower, the bigger bubbles. This provides a visual representation of a message floating to heaven.