I Have a Phone with a 2.5mm Headset Jack and I'd Like to use an Amplifier or a Wireless Headset. Is There a Solution Without Having to buy a lifter?

Absolutely! Use the Male 2.5 to Male RJ9 adapter cord.

Note: The Customer will not be able to remotely answer/end calls when using this setup. This is recommended for Customers who have a 2.5mm telephone base and are always at their desk.