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
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.