Proof of work can't really be done on mobile phones, but using the Federated Byzantine Agreement protocol is. It can be mined on mobile because it's not using machine power to compete against other wallets to mine a block. Nodes come to consensus through agreements in the existence of certain data. You're thinking of proof of work, and proof of work gets exponentially difficult as time goes on. That's exactly what happened with bitcoin. In its early stages, bitcoin could be mined on any computer or mobile phone(if there was a mobile app) as it grew, the PoW protocol turned into something that incrementally grew too complex for regular computers to mine, and today no one can mine it without an immense amount of computing power to even extract a little bit from it.