iPhone 15 Pro Max speaker unavailable after Remote app use

"The reason I can’t make phone calls in this state is that my phone believes it has no speaker or microphone. This has nothing to do with my carrier.: Yes, this is perhaps an AirPlay issue, but the workaround of disabling it means I can’t use AirPlay. Are you suggesting I need to choose between using that feature or the remote app? AirPlay normally works (as well as it does, anyway) under other circumstances without disabling my local speaker until I reboot. It’s the very specific scenario I described above, which is not an edge case, where things go wrong."

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I. So, when did this all begin?

If this has been going on since before iOS26.1, then when did this all occur? If pre iOS26, then it's something you should have had looked in to quite some time ago. It may be a hardware issue, or it may be a software issue. If you were to reset this to Factory Settings (IMPORTANT: back up to a computer, first), then that may fix this. But, before this reset of all things, I recommend you get this diagnosed and serviced.

Getting this Serviced:

A. Take to an Apple store:

You'd setup a Genius Bar Appointment at your local Apple Store, taking it in for service. There, it will be diagnosed, and you'll be provided with your options.

B. Take this to an AASP:

See an AASP (Apple Authorized Service Provider) in your local area...

Contacting an AASP: 

1. Go Here: Find Locations - Apple Authorized Reseller
2. Click: Service & Support
3. Enter: your location information
4. See: if there is an AASP nearby
5. Contact: an AASP that shows up, and find out more about the services that they offer to fix the iPhone

II. Clear the Caches:

Caches are created so that apps open with ease though remembrance of use and interoperability. Use my User Tip: How to Clear "System Data" in your iPhone's Storage: - Apple Community

III. Update to iOS26.2:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update. So, see if an update fixes this. Other than updating through WiFi on the iPhone, itself, you can update using a computer, and even connect the computer using Ethernet, if available, for a more reliable connection. Use my User Tip: Update your iPhone Using a Computer - Apple Community

“iPhone 15 Pro Max speaker unavailable after Remote app use: iPhone 15 pro max speaker unavailable after using remote.”

Steaming seems to be the issue here. So, is AirPlay enabled? If so, turn it off. Is screen mirroring or screen extending on? If so, turn it off with the keyboard: (command + F1) or (command + fn + F1)

“Cannot make or receive phone calls in this state, either. This persists until the phone is rebooted.”

Go Here: If your Call Fails or you can’t Receive Calls on your iPhone - Apple Support

No Success? Contact your Carrier.