I've looked at at the below apps, and can't find the functionality. None of them seem to be able to read the PSBT.
Blue Wallet (there's an issue in github about it)
Nunchuk (they have their own multisig capability, but do not act as a signer)
Blockstream
It would be great if people could use a phone app, specifically to test out and practice with multisig wallets --without needing to buy a bunch of separate hardware wallets.
AI swears up and down that Blue Wallet can do it, but the version mentioned is a much older one, and it's probably hallucinating anyway.