If you want to set up a proper telegram system with D&D tech, Magic Mouth is a better choice. Let's say you set them up onto poles that are spaced 30 feet apart, 4 magic mouths per pole. Say, the line is going east-west:

• Mouth 1: If it hears a "one" coming from the east, it says "one".
• Mouth 2: If it hears a "zero" coming from the east, it says "zero".
• Mouth 3 and 4: same but from the west.

Each pole costs 40 GP to set up, so this telegram is rather expensive, costing 7040 gp per mile... but once it's set up, it doesn't sleep, doesn't need payment, doesn't need maintenance, just two people on each end with a binary code table. You could say that these are skilled hirelings, working in 3 shifts that means that the upkeep of both ends of a line is 12 gp per day.

Peasants shouting the message... well, to make it absolutely sure that the message is heard, you need to put a messaging post every 100 feet. (Loud noises are audible at 2d6×50 feet per the DM screen.) If they are working in 3 shifts, that's 6 sp per day per post, making the upkeep of the line ~32 gp per mile. Thus the magic mouth setup would become cheaper after only 220 days.

Monodrones are probably much more reliable for that. Or you can straight-up use monodrones to set up a proper Clacks system.

With a sufficient number of peasants, sure.