In the Linux Unplugged podcast (Episode 401 "Own Your Mailbox") they said that PipeWire now allows you to use your smartphone as a Bluetooth microphone (A2DP, first 30 seconds of the episode).
https://linuxunplugged.com/401
https://gitlab.freedesktop.org/pipewire/pipewire/-/releases/... ("Make it possible to use an A2DP source as an input device. You can then use your phone as an A2DP microphone, for example.")