Text messaging services are currently available to customers with US numbers exclusively.
If your Voice number is a US number, and you are not able to text, double check the amount of Voice Credit left on your account - you'll need more than $0.50 in credit to text out. You can check, and reload, your credit from this page in your Zendesk Sell account.
Text messages can only be sent to numbers with the same country code and cannot be sent to international numbers.
Our voice provider, Twilio, has several more countries in beta testing for text and voice capabilities. Once they roll these out, you can bet we'll add them to Sell as well.