I've just created my own sms gateway using a Huawei E303 dongle and a cheap GSM sim card and I forsee great possibilities if a CIVI extension can be developed for this. Its based on the OS PlaySMS gateway. PlaySMS itself has an api and can be configured to use many other 3rd party gateways including clickatell and Twillio as well as DIY setups using GSM sim and a dongle. Can all be put on a rasp pi.
I think it would be great if Civi had functionality to use PlaySMS as an SMS provider. It would mean dedicated outgoing and incoming sms functionality available via a cheap sim service. I'd be happy to provide documentation on building the rasp pi setup and configuring playsms, and using the playsms api. https://github.com/antonraharja/playSMS/blob/master/documents/development/WEBSERVICES.md
I'd need support with the CIVI side of things, getting an extension written etc. How to develop this as a MIH? anyone interested in helping?