BigHammer: You forgot Radio! Hardly cost effective, but a non-cellular way to transmit/receive information.
I'd go for the SMS option, maybe mobile data if data usage was small enough. As pointed out above, they're cheap enough to buy.
walkie talkies that can do data? thats a possibility, or make your own radio setup is another way.
or you could look at the harris range of radios and then start using the free HAM radio frequencies. (transmitting on an unauthorised freq is a fine of $10k not to mention the questions of why you have military grade radios)
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