Easyterm's BBS will do what you want, as far as a single BBS server.?? Everyone connecting to your BBS would be using Easyterm and Soundmodem on their end along with their own radio, antenna, etc. . so now it's getting less simple.? Then everyone connecting to the BBS would need their own amateur radio callsign, which sounds to me like it's game over since it would no longer be simple.
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There may be a way of legally sending AFSK data without a license or callsign though, using Part 15 devices, but you'd have to research that.? I think packet would not be legal with FRS/GMRS, and it's definitely not legal with CB or MURS.? You can't even do store-and-forward voice with MURS.? (I say "AFSK", because that's the sort of 'default' and by far the most common data mode used with packet radio.? )
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I think I remember seeing some hacks done to pagers, maybe that's an option?
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Maybe Meshtastic is the simplest method of legally texting over radio without a license.
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Jeff KP3FT