Just about any sms gateway works the same way in regards to a response. They make a call-back to a URL of your choice with the details of the SMS response.
Are you sending from a 'word' or from a dedicated phone number? If you're sending from a 'word', such as your business name, someone can't reply and a two-way integration isn't possible.
If you're replying to a number, the sms gateway can post the details of the reply, such as the phone number and body to the callback URL.
This callback URL hosts an application which pushes the details over the REST API back into Marketo and can look up the person by phone number to associate the right person. (Do you have records with duplicate phone numbers?)
Your smart campaign is listening for a data value to be updated and can act accordingly.
What value are you really getting here? Do your customers like being told exactly what they can respond with ("YES", "NO", "STOP") to a phone number they don't recognise? I'm not saying these aren't useful ways of interacting with your customers, but what about the times when someone replies to an SMS with something other than one of these predefined words? Are you monitoring your SMS responses?
A lot of people will sell you on the dream of a two way integration, but I'd introduce the simple workflow of linking to a landing page/form with a one-way SMS to achieve a similar outcome as an alternative to the effort involved.
.png)