I have been having trouble getting my Bose QC35 headphones working on Linux. The symptoms were:

• A2DP worked fine (high sound quality, no mic - to be expected)
• HFP/HSP did not work at all: no mic or audio output

Now everything seems to work just fine.

“What changed”, I hear you ask…

I bought a new bluetooth dongle. Not a very exciting change I agree, and also does not really narrow down what actually fixed the problem, but here are some working theories:

• The new dongle is bluetooth 5 rather than 4 which means double the bandwidth amongst other changes. The headphones support bluetooth 4.1 and 5 so it’s possible the dongle I bought was 4.0, but unlikely.
• The old dongle was actually defective or broken in some way. This is also unlikely since it worked fine in A2DP mode.
• The new dongle is plugged into a front-mounted USB port, which I know is USB3. The back port may have been USB2 (to be checked).

This post is a living document in the sense of: I cannot check which of the above is the right answer right now, but will do in the future. Watch this space.

Update 2023-07-29: the previous dongle was definitely Bluetooth 4 so it’s likely that was the cause of the issue. Perhaps it was Bluetooth 4.0 which is not supported. It certainly has less bandwidth than Bluetooth 5.