The numbers on the phone pad start with 1 in the upper left corner, going down to 9 in the bottom right corner.
The numbers on a calculator start with 1 in the bottom left corn, going up to 9 in the top right corner.
For example, the top left number on a phone pad is 1. The top left number on a calculator is 7.
This difference makes it challenging to switch between the two. Why were they designed this way?