Roll20 can run on some mobile devices, but the ability to do so requires a subscription. There is an app, but it is only for handling character sheets and dice rolling, it's not meant for map access. The games are hosted and run on this site, but many people use third party programs such as Discord or Skype to handle audio/video, since they are dedicated to that purpose and less prone to failure. Many people handle communication with the built in text chat. There is an extensive LFG system built in to Roll20, but it would be better for someone else to chime in about that; I've never used it. Roll20 is basically a virtual table top. You host maps, tokens, character sheets and other assets, and players interact on the maps and roll dice in real time. For an overview, check out Roll20's Youtube channel or just search Youtube for "Roll20 intro". And lastly, you don't need to subscribe in order to play. Subscription just unlocks better tools and more storage.