Just saw a commercial for a new invisible digital hearing aid study.
Call: 888/718-3310 AccuQuest Hearing Centers
WOW!! They have an awesome website! It's very informative, and they offer quite a variety of models. There is a center close by, which is good if Dad goes with this place because they offer free in-office repairs, cleanings, and adjustments for life.
They don't list prices for any of the models, but mention costs ranging from $750-$4,500 on their FAQ's page.
I will also call the number you provided above. There's no mention of the study on their site, so it seems only possible to get those details by phone.