HFP AG为什么无法建立音频连接？

使用了官方IDF里HFP AG的例程，试着连接了三款蓝牙耳机，能够建立蓝牙连接，但是无法建立音频连接（audio connection）,串口LOG显示如下：

Connect Audio

[0;33mW (42785) BT_BTM: BTM Remote does not support 3-EDR eSCO

[0;31mE (42785) BT_BTM: btm_sco_connected, handle 81

[0;33mW (42785) BT_BTM: BTM Remote does not support 3-EDR eSCO

[0;32mI (42795) BT_APP_HF: APP HFP event: AUDIO_STATE_EVT

[0;31mE (42795) BT_BTM: btm_sco_connected, handle 81

[0;32mI (42805) BT_APP_HF: --Audio State connecting

[0;32mI (42805) BT_APP_HF: APP HFP event: AUDIO_STATE_EVT

[0;32mI (42815) BT_APP_HF: --Audio State disconnected