In a nutshell, a phone headset amplifier is one way of connecting a headset to a telephone. A phone headset amplifier is used for telephones that are not “headset ready”, meaning that the phone does not have a dedicated headset jack. If a phone is “headset ready” it will have a dedicated headset jack that will typically be one of two types, it will either look like the handset receiver jack (modular phone type plug, smaller than the plug from the wall jack to the phone) or it will be a little round 2.5 mm headset jack (looks like an ipod jack, but it is smaller).
The little round 2.5 mm headset jack was initially to only be found on cordless phones and cell phones, but now it has also been incorporated into some corded desk / business phones as well. So if your telephone has a headset jack, you can use an adapter cable instead of a phone headset amplifier. When using an adapter cable, the headset is relying on the telephone for its volume control. Most telephones will only have a volume adjustment for your incoming listening volume (speaker), and not for your outgoing mic volume. Hopefully the telephone puts out enough power to the headset jack to make your listening volume adequate for you, usually the phone type jacks are fine and work good, but the little round 2.5 mm jack is often lacking in power which usually results in low speaking (microphone) volume.
This is where the phone headset amplifier has the advantage. A phone headset amplifier does not just rely on your telephone’s power. It is usually powered by AA batteries or an optional AC wall adapter and has independent adjustable volume control for both your listening as well as your microphone (speaking) level. When using a phone headset amplifier, you will typically need to lift your handset receiver off hook, set it to the side, and then hang it up after each call. If your phone has a little round 2.5 mm headset jack, you can buy an optional adapter cable (model 78333-01) which will allow you to connect a phone headset amplifier through the headset jack instead of the handset receiver jack, that way you just have to press the headset button on the telephone instead of lifting the handset receiver off hook (just like when using a direct connect cable with a modular phone type plug). Although the phone headset amplifier will work through a little round 2.5 mm headset jack with the optional 2.5 mm to modular cable (model 78333-01), a phone headset amplifier is not meant to work through the modular phone type headset jacks. The phone’s make and model determines which device is used, a phone headset amplifier or a direct connect cable. Please check compatibility to see which one is needed for your particular phone.