Tellurium - a WhatsApp addon by Quadren.
Tellurium is a WhatsApp addon that allows you to message people without saving their contacts first. Enter the number into the app along with the country code, and hit "Send". Tellurium will handle the work in the background, and will use the WhatsApp API to redirect you to the correct number!
NOTE: for Tellurium to work, WhatsApp must be installed.