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Other Supplementary Services
The SPA941 supports the following supplementary Services:
Block Caller ID
If enabled, the SPA941 will attempt to hide the caller-ID on all outbound calls by default. The user can
also enable or disable this feature on a per call basis by pre dialing a *code before making the call.
Block Anonymous Call
Reject all inbound calls with no caller-ID with a 406 response
Do Not Disturb (DND)
Reject all inbound calls as if the phone is busy. That is, the phone either responds a 302 if call forward on
busy if enabled, or 486 otherwise.
Secure Call
If enabled, the SPA941 will try to negotiate on all outbound calls to use encrypted media (SRTP) by
default. The user can also enable or disable this feature on a per call basis by pre dialing a *code before
making the call. Note that in order to use Secure Call on an extension, <Mini Certificate> and <SRTP
Private Key> must be configured for that extension.
The enabling and disabling of these services applies to calls on all configured extensions on the SPA941
Secure Call Implementation:
A secure call is established in two stages. The first stage is no different form a normal call setup. Right
after the call is established in the normal way with both sides ready to stream RTP packets, the second
stage starts where the two parties exchange information to determine if the current call can switch over to
the secure mode. The information is transported by base64 encoding and embedding in the message
body of SIP INFO requests and responses with a proprietary format. If the second stage is successful, the
SPA will play a special “Secure Call Indication Tone” for short while to indicate to both parties that the call
is secured and that RTP traffic in both directions are encrypted. The phone screen will be updated with
the CID information extracted from the Mini-Certificate received from the other end. The call state label on
the phone screen will also be prepended with a ‘$’ symbol (such as $Connected instead of Connected).
The second stage in setting up a secure all can be further divided into two steps. Step 1 the caller sends
a “Caller Hello” message (base64 encoded and embedded in the message body of a SIP INFO request)
to the called party with the following information:
- Message ID (4B)
- Version and flags (4B)
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