No sound during Bluetooth calls with SM-G986B and /e/os 3.0.4

• /e/OS version: 3.0.4 -a14-20250709507532-community-y2s
• Device model(s): Samsung S20+ 5G (SM-G986B)
• Impacted Application: phone calls via bluetooth headset

The problem

When making calls via Bluetooth (both headset and hands-free system), no sound is transmitted. The microphone works well. Playing music via Bluetooth is also possible without any problem.

Already tested:

• A2DP hardware offload via ADB disabled -> no improvement
• HD-Audio disabled, SBC-Codec selected -> no improvement
• headset works perfectly with other devices
• microphone is working during a phone call, only the sound does not
• the problem persists even after restarting the smartphone and reconnecting the Bluetooth device
• the setting "Phone calls" in Bluetooth settings is enabled for the headset
• I also checked the different volume controls
• Making phone calls via speakerphone or wired headset is working