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I get your point. But importing Bitcoin private keys to another device/wallet usually isn't recommended. Just create new wallet on your phone and send some Bitcoin to it.
Look at the fees, not really great to waste money on fees when you are looking at current rates, and we aren't talking settling big amounts, but small amounts to use on a phone number, so it's really not worth it.
I get your point. But depending how small is the amount, i find spending an UTXO to one receiving address and pay 24-45 sat/vB (current suggestion by
https://mempool.space/) is barely acceptable option.
Importing and exporting private keys could be reasonable if were you hold your keys is an airgapped device, and you could use a QR code reader so nothing is leaked, and we are talking private keys that don't hold some huge amount so it shouldn't be a problem.
Fair point. My suggestion usually apply to less technical user or when big amount of money involved.