If you received the notification regarding the need of verifying your phone number when you tried to add funds, please follow these simple steps below:
1. In the tab "Payment-->Add funds" click on the "Phone not set, add it here" button
2. You will be redirected to the section Settings-->Account. There, at "Phone", set your country code and your phone number. Fill them in and then click on the "Save changes" button.
3. Go again to the menu Payments-->Add funds and click on the "Send SMS with PIN code" button. Shortly after that, you will receive on your phone an SMS with a code.
4. Insert the code you received and then click on "Verify".
Afterward, your phone number will be verified and you will be able to add funds to your account.