There are two ways that a user can set the agent mobile number to show in dash:

Auto-filled when adding new agents:

1. go to Settings
2. click Setup
3. go to application settings
4. choose New agent default values
5. set the value to True
6. click save

Manually set the agent mobile to show:

1. Search for the agent's record under People menu
2. Cick Agent Basics 
3. Click the plus (+) button next to preferences
4. Select Show mobile phone on profile
5. Click save
6. Click submit