SIP to SMS bridging

I’ve recently updated my local Asterisk PBX. The main reason for the update was that processing the logs in order to set up the firewall rules to block the folk that hammer on it all day long trying to make long distance calls or run up big bills on premium rate numbers was getting too much for the original Mk i Raspberry Pi B (it now runs on a Pi 3 b+ which more up to the task).

As part of the set up I have 3G USB dongle that also supports voice calls using the chan_dongle plugin. This plugin also supports sending and receiving SMS messages (this is different from the MMS/SMS gateway I also run on a separate pi) and they are handled by the dial plan.

My first pass at the dial plan just publishes the incoming message to a MQTT topic and it is then processed by Node-RED, which emails a copy to me as well as logging it to a file.

[dongle-incoming]
exten => sms,1,Verbose(1,Incoming SMS from ${CALLERID(num)} ${BASE64_DECODE(${SMS_BASE64})})
exten => sms,n,AGI(/opt/asterisk/agi-mqtt/mqtt,/opt/asterisk/agi-mqtt/mqtt.cfg,foo/sms,${BASE64_DECODE(${SMS_BASE64})})
exten => sms,n,Hangup()

This works OK for incoming messages but sending them is a bit harder, as the only way to send them from outside the dialplan (from within the dialpan you can use DongleSendSMS) is to use the asterisk CLI tool which is a bit clunky.

What I was really looking for was a way to send/receive SMS messages like you do with a mobile phone. To make calls I normally use Linphone on my tablet and this includes support for SIP messaging. This lets you send messages between SIP clients and you can get Asterisk to consume these.

You can send SIP messages with the MessageSend asterisk dailplan function

The following is a basic echo bot:

[messaging]
exten => _.,1,Answer()
exten => _.,n,Verbose(1,Text ${MESSAGE(body)})
exten => _.,n,Verbose(1,Text from ${MESSAGE(from)})
exten => _.,n,Verbose(1,Text to ${MESSAGE(to)})
exten => _.,n,Set(FROM=${MESSAGE(from)})
exten => _.,n,Set(TO=${REPLACE(FROM,<'>,)})
exten => _.,n,MessageSend(pj${TO},${CUT(MESSAGE(to),:,2)})
exten => _.,n,Hangup()

Because I’m using the PJSIP module rather than the legacy SIP module I need to prefix the outbound address with pjsip: rather than sip:. This also matches any target extension which will be useful a little later.

To enable a specific context for SIP messages you need to add message_context to the PJSIP endpoint for the SIP user:

[tablet]
type = endpoint
context = internal
message_context = messaging
...

Now if we put the 2 bits together we get a dialplan that looks like this:

[dongle-incomming]
exten => sms,1,Verbose(1,Incoming SMS from ${CALLERID(num)} ${BASE64_DECODE(${SMS_BASE64})})
exten => sms,n,AGI(/opt/asterisk/agi-mqtt/mqtt,/opt/asterisk/agi-mqtt/mqtt.cfg,foo/sms,${BASE64_DECODE(${SMS_BASE64})})
exten => sms,n,Set(MESSAGE(body))=${BASE64_DECODE(${SMS_BASE64})})
exten => sms,n,MessageSend(pjsip:nexus7,${CALLERID(num)})
exten => sms,n,Hangup()

[messaging]
exten => _.,1,Answer()
exten => _.,n,Verbose(1,Text ${MESSAGE(body)})
exten => _.,n,Verbose(1,Text from ${MESSAGE(from)})
exten => _.,n,Verbose(1,Text to ${MESSAGE(to)})
exten => _.,n,Set(FROM=${MESSAGE(from)})
exten => _.,n,Set(TO=${REPLACE(FROM,<'>,)})
exten => _.,n,DongleSendSMS(dongle0,${EXTEN},${MESSAGE(body)},1440,no)
exten => _.,n,Hangup()

The first part handles the incoming SMS messages delivered to the dongle and passed to the sms extension in the dongle-incomming context. This logs the message to the console and via MQTT then fires it off to my tablet as a SIP message. The second is the context for the incoming SIP messages from the tablet, this will accept messages to any extension, logs the message, who it’s to/from then sends it to the number in the extension via the dongle.

Using SIP Client to send SMS

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