Since we've switched our telephone line from an PMX line to an SIP trunk we're not able to send SMS anymore. The SMS is send by an server with fax and SMS functionality, which is connected to our PBX via SIP, to the SMS gateway of Deutsche Telekom. As long as the telephone line was an PMX line, everything worked fine. Now it looks like that the the SMS server is not recognizing that the SMS gateway waits for an data transmission, until the SMS gateway answers with an voice announcement, which you can also hear when you call the SMS gateway by phone.
During troubleshooting I've switched the Fax/SMS servers SIP connection to an AVM Fritz!Box, which is connectect to an Deutsche Telekom AllIP line (also SIP, but for privat use), and the server is able to send SMS over this connection. So in generally it works. I'm not very familiar with SIP at the moment. The provider says it's an PBX or SMS server issue, the PBX manufacturer says it's an provider or SMS server issue, and the SMS server manuacture says.....just guess :)
So I did an packet capture on our Fax/SMS server for the SIP connection to the SMS gateway for the working- and the non-working-scenario. According to the captures I've understood the flows like this:
Working scenario (SMS_FritzBox_Working.pcapng)
- SIP call established (G.711A)
- SMS gateway sends an "beep" to signalize, that he's ready to receive an data/SMS transmission
- SMS server answers with an "beep" as well, and transmits data/the SMS
- SMS gateway sends "beep" and data (guess acknowledgement that he received the SMS)
- SMS server sends another "beep" as acknowledgement
- SIP goodbye
Non-working scenario (SMS_PBX_Non-Working.pcapng)
- SIP call established (G.711A)
- SMS gateway sends an "beep" to signalize, that he's ready to receive an data/SMS transmission
- SMS server did not recognize the "beep" of the gateway
- After 10 seconds the SMS gateway response with an audio announcment (according to Deutsche Telekom this is a normal behaviour)
- After another 9 seconds, the SMS server sends a "beep" (like "Hello? Is there someone who want's to talk with me?")
- SIP goodbye after audio announcment
SMS Server (10.3.129.38), PBX (10.3.129.42), Fritz!Box (10.3.129.10) Download
I can't find any differences in the SIP negotioation and the protocol of the both captures. But according to the Wireshark RTP stream graph, the audio volume level of the first incoming "beep" from the SMS gateway in the non-working-scenario is much lower, than the one in the working-scenario. Not sure if this is really the reason for this issue, or if I missunderstood this graph only. But it would be an good explanation for me why our SMS server is not recognizing the SMS gateway. Could something like noise reduction or silence detection be the issue?
I did another capture on the SIP border gateway, the last hop at our side befor the traffic leaves our buildings. The audio volume level of the incomming "beep" from outside is the same as the one that arrives on the SMS server. For me an indication that the audio volume level is already low when it comes in.
Is there anybody with good SIP skills, who can help me to find the root cause for this issue, or maybe also the solution? Or anybody that is having or had the same issue?
