Siemens U15 - Manual (Page 145)

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Security
Security
Line Security
If your phone displays Line Not Secure while you are on a call,
your phone has detected another device monitoring the call.
This message does not appear when caller ID, call cost, or other
information fills the display.
Locking the SIM Card
Your PIN (Personal Identification Number) code protects the
information stored on your SIM card. When the SIM PIN
feature is activated, you must enter your SIM card PIN code
each time you turn on the phone or insert a SIM card. Contact
your service provider for your SIM card PIN code.
Note: You can also activate your PIN2 code, to protect secure
applications such as Fixed Dial.
Find the Feature M > Settings > Security
> SIM PIN
Press To
1
S
scroll to On or Off
2 SELECT (
+
) switch protection on or off
3 keypad keys enter your SIM card PIN code
4 OK (
+
) submit your code