Re: "Techwoven case blocks the microphone and is very muffled speaking quietly."
Commiserations that you are finding the iPhone case causes such difficulty for you.
From the website, it seems the case is supposed to be perfect for iPhone 17 Pro Max:
iPhone 17 Pro Max TechWoven Case with MagSafe — Green - Apple (AU)
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Re: Is this a design flaw or faulty product?
Sorry I don't know for sure. It would seem it is one or the other, as the case isn't working well for you.
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Re: "I need to talk quietly due to thin walls and it is not picked up properly.
Clarity is perfect when I take the case off."
Seems clear that the case really doesn't fit your needs.
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If it's new, you may be able to return or exchange it where you bought it from.
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If purchased from Apple itself: See: Returns & Refunds - Shopping Help - Apple
Eg: says: You have 14 calendar days to return an item from the date you received it.
If you've had it longer and it's a faulty product, it's still worth seeking to return or exchange.it.
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If there's an Apple store nearby: (Apple's store Locator: Apple Store - Find a Store - Apple)
You might want to make a reservation to speak with someone, and show them the case, at the Genius bar.
(It's a good idea to take the receipt, and some ID with you.)
Genius Bar Reservation and Apple Support Options - Apple
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To speak by phone with Apple Support in your country or region:
See: Contact Apple Support - Apple Support
All the best :-)