CoreBluetooth Unknown Error: 708

I have a repeatable scenario where an L2CAP connection between two iPhones fails when activating a Bluetooth microphone that's paired with the same device.

The only clue I'm getting is:

cbncom[53050:2758629] [CoreBluetooth] WARNING: Unknown error: 708

This error number 708 is different from the ones I'm more familiar with (like 431) and has almost no hits when I search online.

Can anyone tell me what this error code is, and/or how to prevent problems generally when BLE/L2CAP must coexist with Bluetooth Audio?