tell the other side that we're using 20 ms packetization for iLBC

- we need to investigate support for refusing offers of 30 ms (like the Nokia
  e-series) or supporting it.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@36623 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -5575,9 +5575,14 @@ static void add_codec_to_sdp(const struct sip_pvt *p, int codec, int sample_rate
 	ast_build_string(a_buf, a_size, "a=rtpmap:%d %s/%d\r\n", rtp_code,
 			 ast_rtp_lookup_mime_subtype(1, codec),
 			 sample_rate);
-	if (codec == AST_FORMAT_G729A)
+	if (codec == AST_FORMAT_G729A) {
 		/* Indicate that we don't support VAD (G.729 annex B) */
 		ast_build_string(a_buf, a_size, "a=fmtp:%d annexb=no\r\n", rtp_code);
+	} else if (codec == AST_FORMAT_ILBC) {
+		/* Add information about us using only 20 ms packetization */
+		ast_build_string(a_buf, a_size, "a=fmtp:%d mode=20\r\n", rtp_code);
+	
+	}
 }
 
 /*! \brief Get Max T.38 Transmision rate from T38 capabilities */