Setting up SMS with Flowroute

We’ve recently enabled some SMS targeted ads, and already people are using it. The thing is, I’m running them through Google Voice right now, and I’d prefer to have it go through FreePBX so I’m not reliant on yet another system, and so I have a little more flexibility in how I want it to behave (as well as the phone number used).

I can’t really find a solid guide on how to do all of this. Flowroute has some instructions, but they leave me hanging at finding an “SMS Callback” (which I don’t know how to find in FreePBX) https://blog.flowroute.com/2016/09/22/receive-an-inbound-message/

Can anyone point me in the right direction?