If you have unlimited (speedlimiting doesn't change a thing) data plan except for throttling you shouldn't be liable for any additional costs (YMMV but the plans that just bog you down to 16/32kbps have a clause you don't pay anything extra until you buy 500mb for 5$ to increase speed or some such). So as long your comm is over internet and not the network you should be safe. Call their customer service if in doubt