If a player with a free account attempts to enter a game I created (pro subscription) with a phone, they can. I can, as a pro subscriber, enter any game I was invited to with a mobile device (phone or tablet) but I have noticed that small screens are not favorable for this. The tab section (contains chat, journals, settings, etc) covers close to 2/3rds or 3/4th of the screen. The toolbar on the left overlays and blocks things from being touched or seen on the map. Touch control on the map for tokens is iffy as some phones don't handle it good. I would say 7 inch screen is probably the smallest you want for a viable usability. Anything smaller and things get too crowded. I recommend if possible to use a tablet as that is officially supported, phones are not.