From the manual:
Configuring the DTMF mode
You want to be able to use your phone to control other equipment or to access certain
services, such as remote polling of an answering machine or telebanking. For these services
you need the DTMF mode.
In the DTMF mode each keystroke generates a tone. If for example you enter a call number
during a call, your call partner will hear the tone signals.
You can specify whether the DTMF mode should be generally activated or deactivated.
DTMF is activated as the default dialling mode.
Press the navigation key to the right or the Menu softkey.
Settings Scroll to Settings and press the Select softkey.
General Scroll to General and press the Select softkey.
DTMF Scroll to DTMF and activate or deactivate with the Select softkey.
➔The setting is stored.
