To change your phone number:
Log in to your customer area
Click on your user tab, on "My information"
Go to the subsection "General Information"
Click the "Edit" button next to the phone field
Modify your prefix if necessary
Enter your phone number
Click on "Send me a code"
Copy the code received by message or click on "Send me a code again" if you have not received a message
Click on the "Save" button
Enter your password and click on the "OK" button
Your new phone number is correctly registered