Hi Steffen,

Have you tried this method?

Accessing Network and Phone Settings

You can view and change many network configuration options and phone settings for the Cisco Unified Wireless IP Phone 7921G by using the Settings menu.

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Note You can control whether a Cisco Unified Wireless IP Phone 7921G has access to the Settings menu from the Cisco Unified CallManager Administration Phone Configuration page. Use the Settings Access field in the Product Specific Configuration section of the phone configuration page.

To access the Settings menu, follow these steps:

Procedure

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Step 1 Press on the Navigation button for (Settings).

Step 2 Use these menu options to view or change settings:

Phone Settings

Network Profiles

Security Configuration

USB Configuration

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Note: These options are configurable; other options are display only.

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Step 3 To select the item that you want to configure or view, do one of these actions:

Use the Navigation button to scroll to the item and then press the Select button.

Use the keypad to enter the number that corresponds to the item.

Step 4 **If a menu option is locked , you must press ** # on the keypad.

From this doc;

http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a008079b4ce.html#wp1328723

Hope this helps!

Rob