A phone number allows you to easily send an SMS message to your device from another SMS-compatible device. Phone numbers are NOT necessary to deliver an SMS via API calls or via the Dashboard - they enable you to send an SMS to your device directly from another cell phone.
To purchase a phone number follow these steps:
1) Click on your device to go to the device's page.
2) Go to your device's Configuration page.
3) Select + Purchase under Configure phone number section.
4) Select the country for the phone number.
5) Click on the Purchase Button.