Calling on the Hivemind

Anyone have good suggestions for phone / app friendly play by post platforms or tools?

The Masks game I play with my daughter and two of her friends hit a real high note with the most recent session, but it looks like we won’t be able to get back to the table until almost mid-August. Need something for asynchronous one-on-one play with a generation that associates “email” with “homework.”

Also, they’re going to be on the road a lot, so something that requires a PC for best experience is not recommended.