It is possible to do what you want, with relatively newer ReSound models, a newer vehicle designed and configured correctly to work with iPhone, and a relatively newer iPhone. Your hardware doesn’t have to be state-of-the-art, but you probably can’t do what you want with gear or a vehicle that’s more than say five or six years old. But as others have indicated, there’s a lot of variation even with newer gear that says it’s Bluetooth-compatible. It may work. It may not. If just one of the three pieces of gear was not made around the same time as the others, your chances for difficulty increase.
For example my iPhone 6 and my ReSound LiNX 2 aids work as you’re seeking in my 2016 Toyota Tacoma and 2017 Jeep Wrangler. All these were made within a couple of years of each other.
However the iPhone only works in the Jeep if I am set as the primary Bluetooth profile in the vehicle. If my wife is set as primary and I’m set as secondary, my phone doesn’t work as you want if her phone is in or near the vehicle. Also my iPhone can play music via Bluetooth through the Tacoma, no problem, but through the Jeep, I have to do this controlling the phone manually. When the truck is running and I step in, it can be set to start playing music from the phone automatically and does a great job. Not in the Jeep. All this stuff is very twitchy and can require trial-and-error, and I wouldn’t expect an audiologist to be able to set this up for you without extra fees.
Also be aware that Bluetooth is a significant security risk. Your vehicle or your phone, probably your aids too (though I don’t know why anyone would bother), can be hacked by an expert bad actor nearby, say one lane away, without a lot of trouble if Bluetooth is enabled.