In phoenix you need enough liquidity in your channel, to receive the amount of sats and later to pay onchain, they use splicing so if you are going to pay onchain the size of your channel will decrease by the amount of sats you are trying to pay. Its not the most recomendable way to do so, just for payment that can fit in your channel liquidity and that will not let your channel empty. Use one of the other tools I mentioned. Boltz, sideshift are super easy and made for this, coinos.io is also super easy, you just send sats to a newly created wallet, and then pay to a btc address. The other options I mentioned, are p2p exchanges, they can be less intuitive to use for this case, but they will maximize your privacy