How do they conduct polls in countries like Kenya and India, where many people don't have phones?
Yeah. Even really poor people have phones nowadays.
In fact, many African cell phone carriers have more functional capacities (bill paying, wire transfers) than US cells phones.
It's like an anecdote I heard from a professor when I was in law school. He visited Estonia, and his host asked to see what a check looked like. He had never seen one before, because they do "everything" electronically. (Probably hyperbole, but the point is salient.)
The US is really quite far behind in infrastructure.