The porting function needs to be enabled first in your account. If you wish to enable it, kindly follow the steps below:

1. Go to Business Settings
2. Click on Plans
3. Click on Plan Conditions
4. Scroll down to the section titled SpherePhone
5. Tick the button next to the line item titled "Number porting" to enable it for all your plans. 

This will show the Port Number button in the customer's account.