For more information on your device, select it on the Support main page.
When in a call and prompted to press numbers to access voice based services, you can press the lock key to unlock the phone first, and select the Dialler command on the touch screen to access the on-screen ITU-T keypad. The Dialler command opens the phone keypad which can be used for sending the desired digits (DTMF codes).
The on-screen keypad allows you to send the required digits for example when calling into a conference bridge or a help desk. When asked to press a number, enter the Dialler menu and press the required button on the keypad.