I don't believe that this issue is with Asterisk.
The issue is that the phone does not set up the RTP stream until it receives the
"200 OK". Asterisk sets up the RTP stream when it receives, or sends, the
message with SDP (either INVITE message, 180 response or 183 response), as per
the RFC.
The firmware for the 91XXi phones were branched
from the 480i version back in late 2004. Since this branching, the 480i firmware
has been fixed, and the phone now sets up the "early media".
The following is from the "KNOWN ISSUES" section of
the 480i release notes from November 2004, v1.0.0.50.
> 2.6 FAILURE TO PROCESS
"REMOTE RINGBACK"
> A device may send an invitation that includes an SDP message. When the terminator responds with an alert message, it many also contain SDP message. This is known as "early media" or "remote ring-back" and indicates that > the terminator will provide ring-back tone. The originating device should provide this tone to the originating user. Currently, the firmware does not process this remote ringback. > Currently under investigation. I would suspect that the 91XXi firmware was never
updated to correct this problem.
Also, The Asterisk voicemail application will
answer the channel if it hasn't already been answered, before playing any
prompts. Answering the channel send a "200 OK" message to the phone. The phone
will then set up the RTP voice stream for the call. So, if the phone is
"clipping" the begining of the voice prompts, it is an indication that the phone
is taking a long time to set up the RTP after receiving the "200 OK"
message.
This behaviour is not seen on the other call
servers that Aastra test the phone against, due to the fact that this issue was
known a long time ago, and these call servers have incorporated a workaround to
deal with it.
Fortunately, there is a workaround for Asterisk as
well. In your dialplan, issue an "Answer" and "wait(1)" berfore sending the call
to voicemail.
Regards,
Derek
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