Hello, did you try to put Android on your Pi and install SMS Gateway app ? SMS Gateway allows you to send SMS by http request, just by getting http://x.x;x.x:9090/sendsms...phonenumber...text...password
I'm really interested by a network SMS gateway, and the easiest way is getting SMS Gateway on a Android device, but don't want to buy an smartphone, and I search a small device like Pi running Android with GSM possibilities.
I'm interested by your opinion on this idea.