XP8: Hold the POWER BUTTON and VOLUME DOWN button at the same time.  You may also watch this Sonim University video to learn how:

 

 

If you need any further assistance with this question, please contact the Sonim Customer Experience Center at 1-833-MY-SONIM (1-833-697-6646) or email us at [email protected]  We are open 24 hours a day, Monday-Friday and we will further assist in troubleshooting.