How do I implement an sms USSD app like this? - sms

Question

I was wondering how to create an SMS app like this where you text something say "1GB" and get some response from server. https://imgur.com/zwFFhzz
Sorry I'm new to USSD so apologies if amateur question. 1) How does an app like this work? What do I need to know to implement this?

Answer 1

To implement a USSD style application you need to have buy in and assistance from the network operator.
There are API's in various toolsets that will allow you to issue USSD commands to the device, these API's simply just pass the request straight through to the GSM modem on the device as an AT command (I can't remember off the top of my head what it actually is), and the modem unit is responsible for actioning it.
Some commands are processed directly on the device. For example if you type in * 06# (or #06 *) i forget which, most devices will display thier IMEI number. Many mobile sites too, will often have lists of "Secret Manufacturer Commands Your Not Supposed to Know" or "Special Commands that get you Free Credit".... :-)
All these are, are fixed strings built into the modems firmware by the manufacturer of the modem, not the device.
Most USSD style SMS's however are actually parsed and acted upon inside the providers network, usually on some kind of application server that's attached to or works in tandem with the main network SMSC's
For example when I worked on the Yalla Bill pay system out in Qatar we routinely had to attend meetings at QTel (The state telcom provider) to explain our routing rules and test them to make sure they where implemented correctly.
When I worked as a network engineer for Orange UK, i would often have to take care of service requests from vending machine companies that used USSD in thier firmware to send messages to a router at the provider's SMSc center.
The other thing to consider too, is that many providers will charge you A LOT OF MONEY to undertake a project like this, even more if you want them to just intercept anything sent to a given number and be routed directly back to you for you to act on in your own servers.
Some providers do have SDK's that allow some kind of similar access, sometimes for free, sometimes for a cost. I remember when I worked for Orange, you could join the "Orange Partners Program" and that got you some nice SDK code and access to various inner network stuff (I built a big chunk of it :-D) that allowed you to do some interesting stuff, since the company became EE and now part of BT however, I honestly don't know if any of it is still in use.
You can also get a shortcode, and while that's not USSD style SMS, you can have SMS messages sent to the code and those messages will then get forwarded onto a TCP server endpoint that you control.
I believe Twilio have this feature available, and I know MBlox do beacuse that's who I use.
It's not genuine USSD, but you can easily set up a shortcode, have folks send SMS to it, then use an SMS send API to reply back to the sending number.
My MBlox account for example gives me a pay as you go HTTP SMS end point, where I send regular TCP based get requests with a set of parameters, and they turn those into SMS's and send them to the recipient. I pay roughly 2p UK per text inside the UK, and it cost me £50 UK to set it up.
On top of that, I got an inbound SMS number, which costs me a fixed £50 per year, and any SMS recieved by the number is sent to a TCP/WEB based endpoint I provide to them, and which I then do what's needed with the recieved SMS when it arrives.
My inbound is a normal full length number though, for a "shortcode" number which is what your picture shows, you'll likley have to pay extra for that.

Answer 2

That looks like it uses a short code for the interaction, the response is parsed to make a purchase of the 2GB via another API.