A:
I don't believe the sound of the phone can be adjusted if you're using the handset. The volume of the ring can be adjusted up or down and if the speaker phone is used, the volume of the speaker phone can be adjusted up or down.
My suggestion would be to contact CapTel and see if they have something that would help you. I purchased one for my father several years ago and the captioning works great for him. Their number is 800-233-9130. You can also view their products on their web-site.
Hope this helps.