In order to hard reset a Telit T91, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Telit T91 device will be brought back to the default configuration of a brand new device.
1. Switch on the phone using Power button.
2. Type in any one of these codes: *#3646633# *#94267357# *#19912006# *#77218114# *#118811# *#881188#. Confirm with Call button.
3. Your phone will now reboot. Well done.