Unable to route phone call audio to bluetooth headset

REPRODUCIBILITY: Always
BUILD ID = OS VERSION: 4.0.1.48
HARDWARE: Sony Xperia XA2 (Dual SIM)
UI LANGUAGE: German
REGRESSION: I didn’t test this headset on the previous SFOS version.

DESCRIPTION:

I am using the “Soundcore Liberty Air 2 Pro” Bluetooth (v5) Headset. I have no issues with the connection and the music playback and also the button controls are working fine.
But unfortunatelly I am unable to switch the audio to this headset during a phone call - the voicecall-ui does not display a button for switching the audio sink device (as it should like described here), this button is shown for another headset but not for this specific one.

PRECONDITIONS:

Aquire a “Anker Soundcore Liberty Air 2 Pro” headset and pair this device with your XA2 (enable the automatic connection for this headset).

STEPS TO REPRODUCE:

  1. Turn on your Bluetooth on the XA2
  2. Turn on the headset (open the case)
  3. Wait for the automatic connection
  4. Start a phone call

EXPECTED RESULT:

The voicecall-ui shall display the button to switch the audio from the earpiece to the connected headset.

ACTUAL RESULT:

The voicecall-ui does not display the additional button to switch the audio onto the connected headset.

ADDITIONAL INFORMATION:

After the headset is connected and a phone call has been established, the journal log contains the following entries:

[root@Sailfish nemo]# journalctl | grep -e bluetooth -e pulseaudio -e ofono
Mär 13 23:22:03 Sailfish bluetoothd[4223]: bluetoothd[4223]: Can't open input device: No such file or directory (2)
Mär 13 23:22:03 Sailfish bluetoothd[4223]: bluetoothd[4223]: AVRCP: failed to init uinput for Soundcore Liberty Air 2 Pro
Mär 13 23:22:03 Sailfish bluetoothd[4223]: Can't open input device: No such file or directory (2)
Mär 13 23:22:03 Sailfish bluetoothd[4223]: AVRCP: failed to init uinput for Soundcore Liberty Air 2 Pro
Mär 14 00:35:49 Sailfish bluetoothd[4223]: bluetoothd[4223]: /org/bluez/hci0/dev_12_34_56_78_9A_BC/fd11: fd(25) ready
Mär 14 00:35:49 Sailfish bluetoothd[4223]: /org/bluez/hci0/dev_12_34_56_78_9A_BC/fd11: fd(25) ready
Mär 14 00:36:26 Sailfish ofonod[3016]: data reg changed 1 -> 2 (searching), attached 1
Mär 14 00:36:26 Sailfish ofonod[3016]: Deactivating context: 1
Mär 14 00:36:26 Sailfish ofonod[3016]: Deactivating context: 1
Mär 14 00:36:26 Sailfish ofonod[3016]: Clearing active context
Mär 14 00:36:26 Sailfish ofonod[3016]: Deactivated data call

Some device information:

[root@Sailfish nemo]# bluetoothctl
Agent registered
[Soundcore Liberty Air 2 Pro]# devices
Device 12:34:56:78:9A:BC Soundcore Liberty Air 2 Pro
[Soundcore Liberty Air 2 Pro]# info 12:34:56:78:9A:BC
Device 12:34:56:78:9A:BC (public)
        Name: Soundcore Liberty Air 2 Pro
        Alias: Soundcore Liberty Air 2 Pro
        Class: 0x00240404
        Icon: audio-card
        Paired: yes
        Trusted: yes
        Blocked: no
        Connected: yes
        LegacyPairing: no
        UUID: Serial Port               (00001101-xyz)
        UUID: Audio Sink                (0000110b-xyz)
        UUID: A/V Remote Control Target (0000110c-xyz)
        UUID: A/V Remote Control        (0000110e-xyz)
        UUID: Handsfree                 (0000111e-xyz)
        UUID: Vendor specific           (66666666-6666-6666-6666-666666666666)

According to the product’s webpage this headset supports the bluetooth profiles AVRCP1.6, A2DP1.3 and HFP1.7.

Could someone please point me to some further ways of debugging?