Will they ever make a cellphone that's hearing aid "compatible"?

Well, step one: Don’t answer your cell phone in the car. There’s nothing in the world that can’t wait until you pull over safely. You of all people should know that. No one should answer their phone in the car.

Next, does it help to put the phone on speaker? I understand this isn’t the ideal solution, but perhaps it would work in the interim?

Third, AT&T has a rating system for hearing-aid compatible phones on their network (I remember you saying you’re on T-Mobile… I’ll look for one from them as well.)
here

Here‘s the T-Mobile list – about ⅔ of the way down the page.

Aren’t there hearing aids with mini jacks for iPods? I’m sure you’ve considered that. Just a thought.

Here are CNET reviews for hearing aid compatible cell phones.

I think they’re going to have to!

There are Bluetooth + Hearing Aid devices available. They pair with your phone and perform normal hearing aid duty the rest of the time.

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Pure conjecture on my part, but I’d think you’d ideally want something like an in-canal hearing aid that’s able to talk to your cellphone over Bluetooth. It’d act as your all-day hearing aid most of the time, but if you answer the phone it would handle the ear-piece function of the phone. You’d either use the phone’s builtin microphone, or even better, the device in your ear would have some spiffy tech to pick up your natural voice.

Seek, thanks

Yes. One of the guys in my group is working on a low power transmitter that works directly with cochlear implants. It uses the body as the antenna/pickup.

My mom just got a hearing aid that it bluetooth compatible. Then she got a cellphone with bluetooth.

Ha! What @robmandu said—somehow I missed that.

What problems are you having with the hearing aids and your cell phones?
This is a complaint I almost never hear anymore, as most of my patients are able to home any phone up to their hearing aids with little-to-no feedback.

I am a Dispensing Audiologist and work with the three top manufacturers with Bluetooth hearing aids and accessories… but as I said, most of my patients don’t even need them. Contact me here or through PM with more details about your make and model of hearing aids and degree of hearing loss and I’ll gladly help you out.

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P.S.
As a police officer, you already know that it is dangerous to use a hand-held cell phone while driving regardless of whether you wear hearing aids. Please practice what you preach—not only to serve as a true role model, but also because there are people who care about you.