I actually did this at one point while I was still using a pool. However, now I have my own P2pool set up so I could do it through that instead.

Is it impossible on a phone with BTC? Shouldn't be

Monero mining is currently more decentralized yes. But the infrastructure to mine it is available at a low cost to large corporate actors if it ever became very high in market cap or if the government wanted to coordinate an attack against it. There are various tradeoffs and it's not quite that simple, in my understanding anyway.