To amplify the in call audio consider the Plantronics M22 amplifier. The customer will need to use either batteries or AC adapters for it to work.
The Plantronics M22 not only amplifies the incoming signal but it also compresses the loud signals. The advantage of this is that once the gain is set, all the calls will come in at the same level. The agent doesn’t need to adjust each call. The circuit is basically an AGC (automatic gain control).
On the transmit side, the M22 also has a transmit expansion circuit (voice switch). When the headset user isn’t speaking, the transmit circuit shuts down so the end user won’t hear the background noise. However with that being said, if the background noise is close to the loudness of the users voice, the voice expansion circuit will not work as it cannot distinguish between the speaker’s voice and noise.