Try this... it "might" work. I've had it on where I have music from phone going to portable bluetooth speaker, but phone calls sound go to phone... I don't want my phone call sound going through a music portable speaker, although you could do that...lol
So, give this a try:
Settings/sounds & vibrations/separate app sound/select phone app/ >>>> Turn on...
Under app, pick "phone";
Under Device, select "bluetooth" headset.
I'm assuming/hoping it will keep your phone microphone working like you want... "never tried this".. either this or some variation of it "might work"... good luck!
edit: I just tried w/ Samsung earbuds, and it does NOT seem to separate the microphone from the bluetooth earbud...? So, let me know if you have any luck with this idea.
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you. - Thanks!