Aastra 6755i User Manual page 268

Lock the Phone
2. In the "Lock the Phone?" field, click
The phone locks dynamically and displays the following message:
"Phone is locked".
Unlock the Phone
3. Click on Operation->Phone Lock.
4. In the "Unlock the Phone?" field, click
The phone unlocks dynamically and displays the following message:
"Phone is unlocked".
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Model 6755i IP Phone User Guide