I use a JBL Flip4 for music and phone calls throughout the day. I also use a Jawbone earpiece for calls occasionally. For calls only the JBL (and Jawbone) recently started switching from the bluetooth device to the S9s speakerphone for apparently no reason. A couple of more points:
I've tried doing things like disbling my music playback app, restarting the phone (of course) and nothing seems to help.
I'm experiencing the exact same issue. Ironically it plays music no problem via any bluetooth device. It's only phone calls it will randomly swap to speaker on my cell phone but still show it in bluetooth mode. I have to turn it off and on to get it reconnected. Super frustrating when I'm in my call or in a meeting. I feel bad now because I returned a bluetooth headset thinking it was the headset but learned later it was any bluetooth device.