Cell Phone choice

If AT&T offers the Samsung Convoy2, I think it would fit your requirements.
It meets mil specs for drops and water resistance. Battery life is at least two weeks if left on 24/7 unless you use a lot of talk time. Don't know about the unlocked part though. I have one from Verizon.

Have a look at the Moto G or the Nexus 5, though 'smart' they are unlocked and (relatively) cheap.
Lots of freebies that you will not need computers and/or 'pads' to carry out.
After years of living with a dumb phone I bit the bullet and purchased a Moto G. I have a pay as you go plan that I rarely use now. With the Fongo app I can make/receive calls for free.
The Gas Buddy app alone will save bundle for travellers like us.
If you worry about privacy then keep clear because you will be sucked into the Google ecosystem.

Samsung Rugby III..... Very stout and good performing phone.

For the past three years I have had a Casio Ravine 2 flip phone. It was, by far, the best phone I ever had. I, like you, am hard of hearing and it had a great speaker that was loud enough to hear even while driving my deisel truck.
It is also a very rugged phone that meets military specs for water, dropping etc. and it has a good camera.

I just recently changed to a Samsung S4 smart phone which I like very much but am keeping that Casio for backup.

I am on Verizon and am not sure about the compatibility of the phone with AT&T.
Barney

I just changed phones. Started out with a Motorola and exchanged it 10 days later. Found it did not have the power of the old LG I had. Now have another LG but also considered a Samsung. Both seems more sensitive then the Motorola.

Samsung still has a ruggedized flip phone that is pretty cheap and works good on Verizin network

Go into the store for the carrier you want and see what they have.

The choices change all the time, but no longer very many non-smart phones on anything other than pre-pay plans. Many of the non-smarts are special texting phones, but you can still get basic flip phones. I'm currently seeing Samsung, LG and Pantech at the online store (AT&T has been a big customer for Pantech).

Alternative is to pay full price third party for an unlocked GSM phone and take it to AT&T for a SIM. They'll do that now, "BYO" is one of their two ways of getting out of the business of burying phone costs in the service fees (other is outright sale on installment plan).

I'm currently carrying a LG440 as a backup phone (GSM through Tracfone) which works fine with my bad hearing. You should be able to find something like it. The LG 380 is close to being the same phone. LG 380 has advantage of Bluetooth to drive a headset.

For me the LG replaced a Moto C139 (which I liked better because it was a tiny soap-bar phone) that I had to give up when I could no longer hear it ringing.

Samsung Rugby III is a great basic flip phone with a good camera, somewhat ruggedized, but a bit on the heavy side, and expensive ($270 outright, $99 paying through a two-year plan).

I only know this from numerous reviews - no personal experience with this phone. The Samsung T159 is supposed to be the best non-smartphone, flip phone. Call quality is said to be excellent. It has a camera. It has some features. Do a search and look for professional reviews of the T159. I am not sure about how loud it is or what can be done to increase volume - but if you find the Samsung webpage for this phone the manual will be there to tell you what adjustments can be made on it - especially for call volume.

We got two zte valets smarties. Wife has no desire to run a smarty so when I set it up I took off all the fancy stuff. Only one screen, none to flip through. Data shut off. Just basically a phone with texting as needed and a camera. Still has problems but is getting there.
We went Tracfone and the smarties have triple minutes. Told her if she wanted a flipper again it would be less minutes for the same price. She agreed to learn it.
I don't use mine for a computor, cripes I gotta iPad and a laptop and you can actually see the screen. Have my data shut off too. Oh, it has wifi if needed.

I know you said you don't want a Smart Phone, but with your minimal needs, especially the camera, you would never need a separate camera due to the high quality of the photos. in addition, there are so many wonderful features on smart phones, you may not need a computer.