Anyone can install WhatsApp on a phone and enter your number on the verification screen. You will then receive texts and calls from WhatsApp with the six-digit code.

Enter the number located on your gift card or wallet code and click "Continue" to verify that you are redeeming the code to add funds to your Steam wallet.

Where Do I Enter the Verification Code?. Google offers a number of free services such as Gmail, Adsense and Google Docs that require a single Google account to access.