Before sending an SMS to your contacts, it is strongly recommended to preview it and to send a test. You have 2 options for the test sending: sending to a personal number or sending to test group.
Single Test
You can send the SMS to your personal or professional mobile to see the rendering. In the Test interface, indicate your phone number and select a contact for the preview:
You will receive the message with the same view the contact you selected would.
Warning! Do not answer STOP to this message to test the unsubscribing! It will unsubscribe the selected contact for real.
Test Group Sending
To use this option, you must create a test group as you would for an email. The difference is that you must target contacts with a phone number who are admissible to the SMS channel. You can verify the admissibility of contacts in their profiles: 
Make sure to select contacts from your company who agree to receive your tests.
To send the test, go to the Send interface and select the test group as a recipient.