I've actually been trying (unsuccessfully) to do the opposite - ditch the voice plan, and have data only on my smartphone. I can't find any ways to do that either.
This is very easily done, you just aren't looking at the right carriers.T-mobile will give you the data only (5GB 4g then throttled after that) for the 30 dollars a month. But they also are nice and throw in 100 min voice just in case you need it. Oh and unlimited texts.ATT and verizon are the best carriers as far as national coverage, and the worst carriers as far as value and customer flexibility/options.