Experiencing enormous noise when initiating Bluetooth headset call mode on iPhone When I'm initiating a call using any 3rd party app like Slack or Google Hangouts using Bluetooth headset (I've tried a couple of different models from Bose) on my iPhone I'm experiencing very loud noise.

When I'm connecting headset during the call or listening music it just works as expected.

Initially, I was blaming defective headset, so I tried few different models, and it's the same.

Did anyone experience something like that?
I had the exact same problem and reached out to Slack support.   Got this reply

Thanks for writing in with those details, and sorry you're not able to make any hands-free Calls at the moment!
Unfortunately we don't yet support bluetooth connectivity on iOS devices -- that's only available on desktop for now.
That said, full support is in the works! I don't have an ETA to offer right now, but please trust that the team are working on freeing up your hands!

So surprisingly it looks like bluetooth support is app specific not iOS specific.