I think you're onto something here. Did the issue ever show up again for you?
I had the exact same issue with my GS4 and my 2013 Hyundai Santa Fe Sport. Disabled OK Google Everywhere on all screens and lock screens as well as the audio history and then restarted my phone. After that I couldn't make the problem happen again. Then I renabled the OK Google Everywhere settings as I had them before and restarted my phone again. After 2 or 3 calls the issue returned. Once more I disabled the OK Google Everywhere settings and restarted, made about 10 consecutive calls with no issues after that.
I've read elsewhere that other people have had similar success resolving the issue by disabling S Voice. My guess is that on the Samsung phones at least, having both S Voice and OK Google everywhere trying to listen while bluetooth is on causes some type of contention issue for the bluetooth. Of course, that doesn't explain why it accepts the command and dials properly, and the sound only drops out once the connection is made. Very odd, but I'm hopeful that for now at least, this issue might be resolved for me.