Fun Fact: Apple for some reason still lists the Teletype Adapter on their store, with photos of the iPhone 3GS
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Apple iPhone TTY Adapter
https://www.apple.com/shop/product/MA854G/A/apple-iphone-tty-adapter
Apple iPhone TTY Adapter lets you use your iPhone with standard TTY devices, compatible with all iPhone models. Get fast, free shipping when you shop online.
From what I can tell it has been out of stock since roughly 2019, but the page is still up. I fell down this mini rabbit hole starting from this page.