That’s a better explanation.
DTMF may be useful, but since you’re looking for more capabilities, you may like to rephrase your post’s title and ask for a more specific solution.
LoRa or other low power radio networks might be better suited.
Even 3G/4G data might offer a good package at reasonable cost.
Btw, DTMF is a VERY low rate signalling method - no use for voice quality.
Read up on FSK for the basics of low rate data over low quality audio mediums.
You can put DTMF on a voice channel, but not thee opposite.