Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If you have raised volume on both handset and base station, to their maximum, and need it to be louder, you may need a special handset with a 'amplified' capability, I think.
In short, you may need another, louder, phone. There may be no other way to raise the volume on your current phone model, regrettably.
But if you had something else in mind, let me know your thoughts on the matter, and I will advise you to the best of my ability.