(iPhone 18 Pro, iOS 27.0) Muffled audio system-wide when on call

Expressing or conveying auditory experiences effectively through these forums may be challenging. The underlying sound issue on your iPhone cannot be accurately diagnosed from a YouTube link. Factors such as your iPhone's specific sound settings, visual defects, and potential hardware problems cannot be assessed through this method. For more personalized and in-depth assistance, I recommend securing a Genius Bar Reservation and exploring the various support options provided by Apple.

Get a Genius Bar Reservation and Apple Support Options - Apple

When you encounter issues with your Apple devices, the Genius Bar at Apple Retail Stores is the go-to place for expert assistance. The Genius Bar is staffed by knowledgeable Apple employees, often referred to as "Geniuses," who can diagnose and resolve a wide range of hardware and software-related problems. To ensure personalized attention and a convenient visit, Apple provides the option to schedule a Genius Bar Reservation.

Contact Genius Bar.

Manage Reservations - My Reservations - Apple

I'm having the same issue. Upgraded from an iPhone 15 Pro to an iPhone 18 Pro on Verizon, and I never had any call quality issues with my 15 Pro. Now, most of my calls sound muffled, like a bad, narrowband landline call. FaceTime Audio with the same people is crystal clear, so it doesn't seem to be a microphone or speaker issue.

I've talked to Verizon multiple times. They first did a network refresh, then gave me a new eSIM. Both seemed to fix the problem temporarily, but the muffled call quality returned about a day later. I've also reset network settings and toggled Wi-Fi Calling, 5G Standalone, and Voice over 5G Standalone on and off.

Apple escalated my case, and I submitted a bug report through apple.com/feedback. Verizon also offered me a replacement phone, which I eventually accepted. The first day with the replacement, calls were crystal clear. By the next day, the muffled audio was back.

Interestingly, doing a hard reset seems to restore clear call quality, but the problem eventually returns. I'm wondering if it might be related to connecting to Bluetooth or Apple CarPlay, although I haven't confirmed that yet.

At this point, I'm wondering if this is a software issue that Apple can fix with an update. It's incredibly frustrating to buy a new, premium, expensive phone and not have basic functionality like making a clear phone call.

Has anyone else experienced this, and has anyone found a lasting fix?

What Call? On an iPhone, you can make and receive various types of calls. Here are the different types:

1. These are standard voice calls made over the cellular network. They utilize your carrier's voice network to connect you with other phone numbers. Allow phone calls on your iPad, iPod touch, and Mac
2. FaceTime Audio allows you to make voice calls over Wi-Fi or cellular data using your internet connection. It provides higher audio quality compared to traditional cellular calls.
3. FaceTime Video enables you to have video calls with other FaceTime users. It uses the front or rear camera on your iPhone to transmit live video during the call. Use FaceTime with your iPhone or iPad - Apple Support (IN)
4. VoIP (Voice over Internet Protocol) calls use internet connectivity to make phone calls. Apps like Skype, WhatsApp, Viber, and Google Hangouts utilize this technology, allowing you to make voice and video calls over the Internet.
5. Some carriers support Wi-Fi calling, which allows you to make and receive calls over a Wi-Fi network instead of relying solely on cellular coverage. This can be useful in areas with weak cellular reception. Make a call with Wi-Fi Calling - Apple Support (IN)
6. The App Store offers numerous calling apps that utilize different technologies. These apps can provide additional features and options for making calls, such as international calling, call recording, and more.

It's important to note that the availability of certain call types may depend on your iPhone model, iOS version, and carrier.

Same problem here, and you're not the only one:

https://www.reddit.com/r/CarPlay/comments/1wlsgaq/carplay_phone_calls_sounds_much_worse_on_iphone/