You can indeed use SMS and MMS (approx 300kB size).
Or you can use gprs.
Two choices with that.
- Have the device dial-in every x minutes, easy, you just need a server to call to. No size limitations and it's deterministic and flexible. (SMS can get delayed a lot)
Plus you'll get the option to ask for a long term support server contract. $$- Have the device listen, difficult, most networks don't support this since they use NAT, you need special expensive contracts from ISP's
![ThumbsDown :--](https://www.eevblog.com/forum/Smileys/default/icon_thumbsdown.gif)
giving you an internet ip instead of intranet ip. Also keeping a session alive forever is not appreciated by ISP's. But you don't need a server.
Looking at reliability, you should find some PLC supporting gprs modems from a good vendor. And read the manual before, you must be able to configure it by rs232 or usb. I think those SMS configurable things are awful.
Die hard method: encode your data to sound and use a voice line, most realtime and most reliable since calling is was the main goal of the network.