Recently, when my wife and I went on a trip to New York, we wanted our son to be able to SMS us without incurring the extra high costs associated with international SMS messages. I looked around for solutions to the problem and found nothing that did what I was looking for. Here’s what I wanted to do:

  • I wanted my son to be able to SMS me for free.

  • I wanted to be able to know my son had SMSed me, and be able to respond to him for free.

My wife and I both have Android based phones. I have a little bit of skill in trying to write Android applications. Since it is Java, which I’ve got a lot of experience with, I have been able to write small applications that seem to work. I figured I’d be able to leave my wife’s Android phone at home attached to WiFi and make it into a gateway for SMS messages.

I have found that Direct Messages do NOT currently support the new Annotation feature. This is a real problem. Because of this, I don’t believe I can use this feature for my implementation and will have to look at other ways of doing this.

So I set out to design and implement something that would allow SMS messages to be forwarded to somewhere on the internet, and then when responded to on the internet would cause the gateway to send those SMS messages back to the original sender. But what would I use for the ‘internet’ portion of the application? I could have written a small TCP/IP socket server to do this, or, use something that is already on the internet and running 24/7. I chose Twitter as the forwarding service. So the application was starting to look like the following:

  • Sending phone sends an SMS to gateway phone.

  • Gateway phone recieves SMS message.

  • Gateway uses a preconfigured Twitter account (the SMS forwarding account) to Direct Message another Twitter account (the receiving account).

  • Recieving Twitter user sends a reply to the Direct Message

  • Gateway detects the reply to the direct message.

  • Gateway forwards the reply text as an SMS to the original sending phone number.

  • Sending phone receives the SMS from the gateway phone.

So, there are a few problems with this that don’t seem to be easy to solve with the current Twitter APIs. First, the gateway must poll for DM replies. Second, the DM messages do not carry any information related to exactly which message is being replied to. This is where Twitter’s new Annotation feature (due to be released soon) may come in handy.

Using Annotations, the original sender of the SMS message could be tagged to the Direct Message. I would hope that the annotation would also be passed on to the DM reply when the user replies to the DM, however it may be up to the twitter client to be able to handle the annotations in this fashion.

If that is the case, it may be necessary to write a specialized twitter client that knows how to handle the annotations, and makes sure the annotation is copied from the original DM and passed to the reply DM so that the gateway knows who the message is intended for (knows the original SMS phone number).

During the Twitter Annotation Hackfest

My first question at the hackfest was whether Twitter Annotations are carried on Direct Messages. The answer was a resounding No. So, I needed to rethink the initial design ideas, and take a step back, coming at it from another angle.

So, Direct Messages wouldn’t help me. How could I do a very similar thing using just Twitter Status messages. Accounts can be marked so that their timelines are private. This is of course optional. No matter, the content of a stream is like a subscription. Why not just post the SMSs coming into the gateway as status updates. Then if we annotate each status update with information about the original SMS phone number, we can direct replies back to the original phone.

The only drawbacks to this are that the replies will now be public unless both user accounts have a private timeline, and the phone number is also available if the SMS forwarding account has a public timeline. The latter can be solved with some encryption of the phone number information.

What does this look like now?

  • Phone sends SMS message to the gateway Android phone.

  • Gateway Android phone updates a Twitter status for a special SMS account. Applying an annotation referring to the sender phone number

  • Any twitter user subscribed to that SMS account’s twitter feed will be notified of the new SMS in their friend stream.

  • Any user can reply to the status updates. As long as the reply is valid, the SMS Gateway program will detect them.
  • SMS Gateway program polls for replies until 10 minutes of inactivity occurs.
  • When a new reply is detected, the original status message is fetched from Twitter, and the annotations are used to determine where the SMS reply needs to be sent.

  • SMS Message is sent to the original sender.

In this way, we have established a way for SMS to get into Twitter and be responded to by anyone subscribed to the Gateway’s twitter account. This could allow for a ‘help desk’ like model. But of course the original idea was to provide a way so that international SMS charges could be avoided for people with internet access.

Of course, the application developed at the hackfest will only be useful if Twitter releases their annotation feature as planned, sometime later this year. I’ve had a great time at the hackfest and look forward to seeing the demonstrations.