After a (admittedly possibly sizeable) investment you could send free, simple, international, text messages, from a mobile device, forever.
The idea is a machine that takes a number of pay-as-you-talk sim cards (maybe 16 or 32 would be ok), and communicates with the relevant mobile network as if it
were 16 or 32 individual phones.
The machine has a little keyboard and text display. You need two of them.
The free principle is that calls are not charged for at all until they connect. When you type in your message and press send, machine 1 communicates with machine 2 in binary (i = one ring and hang up / o = no ring).
Certainly it`s going to be cumbersomely slow, especially with international call connect times. Also you might need some tricky error correction programming, especially as communication is essentially one way at a time. Certainly you could not make a product of it, as the involved companies would stop it somehow.
That said, mobile networks in the UK have do have agreements that allow their pay-as-you-talk customers to roam internationally, with no additional call charges except those incurred when a call connects.
also I reckon it`s something that someone a little more technically minded than myself could cobble together with readily available parts.