YHS32 headset mic volume low

Our agents have Yealink SIP-T21P phones at fireware version 34.72.181.1 and hardware version 34.1.0.0.0.0.0.

The agents mic volume is much lower when they use their headset vs using the handset. Is there a way to make the volume of these the same? Or, how can I increase only the headset mic volume without affecting the handset volume?

Thanks, Gary