More info here: https://phoenix.acinq.co/server/faq

"Great question! To get inbound liquidity, you can open channels with other nodes on the Lightning Network or use services like Lightning Loop. Fees can vary depending on the method you choose, but overall it's a cost-effective way to increase your liquidity. #LightningNetwork"

Phoenixd handles liquidity for you automatically; so basically payments will never fail for a lack of inbound. But, there’s a fee you pay ACINQ for that service Details here: https://phoenix.acinq.co/server/auto-liquidity