A good idea. In most countries, people can only buy themselves a mobile phone. They do not have the money for a full-fledged computer.
You don't get the scenario of using phone as crypto mining in this regard. Although this thread isn't giving enough information about how phone will be used as Crypto mining then you can find it elsewhere.
Samsung has started to build a dedicated crypto mining through old models Samsung S5s, they built not with one but with a number of phones bind together and produce each phone their maximum power and combined it to accommodate enough hash power in order to qualify as crypto miner that phone can offer.
This setup was released to public and currently they're remodeling and rebuilding more reliable configurations to prove that these old phone are actually able to mine cryptocurrency in parallel with more expensive high-end dedicated miners today.