Aastra 9000i Series Administrator's Manual page 258

If an SRTP enabled IP phone initiates a call, and the receiving phone is also SRTP enabled, the IP
Phone UI displays a "lock" icon, indicating that the call is secure. If the receiving phone does not
support SRTP, the IP phone will send unsecured RTP messages instead of SRTP encrypted
messages. However in this case, the IP Phone UI does not display the lock icon - indicating a
non-secure call.
You can configure SRTP on a global or per-line basis using the configuration files or the Aastra
Web UI.
Silence Suppression
In IP telephony, silence on a line (lack of voice) uses up bandwidth when sending voice over a
packet-switched system. Silence suppression is encoding that starts and stops the times of silence
in order to eliminate that wasted bandwidth.
Silence suppression is enabled by default on the IP phones. The phone negotiates whether or not to
use silence suppression. Disabling this feature forces the phone to ignore any negotiated value.
You can configure silence suppression on a global-basis using the configuration files or the Aastra
Web UI.
Configuring RTP Features
Use the following procedures to configure the RTP features on the IP phone.
Configuration Files
For specific parameters you can set for RTP features in the configuration files, see Appendix A, the section,
"RTP, Codec, DTMF Global Settings"
IP Phone UI
Step Action
1 Press on the phone to enter the Options List.
2 Select Administrator Menu.
3 Enter your Administrator password.
Note: The IP Phones accept numeric passwords only.
4 Select SIP Settings.
5 Select RTP Port Base to change the RTP port base setting. Default is 3000.
6 Press Done (2 times) to save the change.
Note: The session prompts you to restart the IP phone to apply the configuration settings
41-001343-01 Rev 02, Release 3.2.2
Note: If you enable SRTP, then you should also enable Transport Layer
Security (TLS). This prevents capture of the key used for SRTP
encryption. To enable TLC, set the Transport Protocol parameter
(located on the Global SIP Settings menu) to TLS.
on page A-124.
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