PhoneLineTransportDevice API no longer works on Windows 22H2

[BestOwl]

Recently I discovered that starting Windows 11 22H2 (build 22621), PhoneLineTransportDevice API no longer works, which resulted in our app's Calling feature no longer working.

The Calling feature currently relies on the PhoneLineTransportDevice API. The following is a brief description of how to use this API:

1. Enumerate a PhoneLineTransportDevice using either the DeviceWatcher or DeviceInformation.
2. Call PhoneLineTransportDevice.RegisterApp() to register the app with Windows for the associated PhoneLineTransportDevice.
3. Call PhoneLineTransportDevice.ConnectAsync().
4. Use PhoneLineWatcher to enumerate PhoneLine, similar to the DeviceWatcher.
5. Call PhoneLine.Dial()
   Note: Microsoft's Your Phone (now called Phone Link) uses the same mechanism to implement the Calling feature as of 2020, not sure for now.

However, after upgrading to Windows 11 22H2 (build 22621), calling PhoneLineTransportDevice.RegisterApp() either results in non-functional or throws a System.UnauthorizedAccessException, even though PhoneLineTransportDevice.RequestAccessAsync() returns "Allowed".

I tested Phone Link on my PC and verified that it works without any issues. Therefore, I am not sure why our app is experiencing this problem.

I am seeking assistance in resolving this issue. If the PhoneLineTransportDevice API is no longer usable, I am open to suggestions for alternative solutions.

[BestOwl]

Windows 10 22H2 also has this issue.

[BestOwl]

New discovery on Windows 11 22H2: In UWP apps, PhoneLineTransportDevice.RequestAccessAsync() now will always return "DeniedBySystem"

[BestOwl]

I made a small UWP app to test this API.
https://github.com/BestOwl/MyPhone.CallingDiagnosis.Pltd

Here are the test results:

OS	API status
Windows 11 22H2	❌ PhoneLineTransportDevice.RequestAccessAsync() return DeniedBySystem
Windows 10 22H2	❌ PhoneLineTransportDevice.ConnectAsync() return false, unable to connect
Windows 10 21H2	✅
Windows LTSC 2021	✅

[BestOwl]

Found this method in YourPhone.Calling.Managed.dll from the latest PhoneLink app:

public static unsafe bool TryUnlockPhoneLineTransportDeviceAPIs(IObjectReference _obj)

Seems like Microsoft blocks this API on purpose🤔.

[abusaadp]

Has Microsoft made an official announcement that Bluetooth calling will not work in Windows 11? Or have they changed the API for Bluetooth calling?

[abusaadp]

I contacted Microsoft and they said they were not aware of this issue. After checking they said there is some permission issue with this API in Windows 11 and they will resolve it soon. So we must wait for Microsoft to resolve this issue. Thats why this API is working for PhoneLink app as it may not have this permission issue.

[abusaadp]

Recently I got an email from Microsoft saying that this issue has been resolved but still the app is not working for me. The windows version has been updated to 23H2. Also even in the previous version of Windows namely 22H2 the bluetooth calling was working for the following apps.

1. Grid3 (https://thinksmartbox.com/product/grid-3/)
2. CallCenter (https://www.justremotephone.com/) and
3. Thy Phone (https://apps.microsoft.com/detail/thy-phone/9PKT9JS17KXZ?hl=en-us&gl=US)

So few of the apps bluetooth calling is working for 22H2 itself. Maybe there is some issue in our app.

[Manmenhongyi]

I have encountered such a problem. After compiling, packaging and installing, I can't make a phone call. When I press the dial key, the software will crash.😰

[Manmenhongyi]

I can't use the dial-up call function normally in Windows10 22H2 and Windows11 23H2.😩

[zhenbohuang]

@BestOwl

I try CallCenter (https://www.justremotephone.com/) and Intel Unison in 22H2, and both them worked fine.

And i find that both them pair the phone in app, but not from OS.
And if you pair the phone from OS firstly,
you need pair from the app again, or unpair from OS then pair again from the app before you can add the phone to the app.

Maybe it is the reason of why PhoneLineTransportDevice.RegisterApp() throws a System.UnauthorizedAccessException,
because the phone is NOT pair by the app, so the app do not have enough permission to link it?

[BestOwl]

@BestOwl

I try CallCenter (https://www.justremotephone.com/) and Intel Unison in 22H2, and both them worked fine.

And i find that both them pair the phone in app, but not from OS. And if you pair the phone from OS firstly, you need pair from the app again, or unpair from OS then pair again from the app before you can add the phone to the app.

Maybe it is the reason of why PhoneLineTransportDevice.RegisterApp() throws a System.UnauthorizedAccessException, because the phone is NOT pair by the app, so the app do not have enough permission to link it?

I tried paring the phone in-app, but still no luck.

[BestOwl]

Recently I got an email from Microsoft saying that this issue has been resolved but still the app is not working for me. The windows version has been updated to 23H2. Also even in the previous version of Windows namely 22H2 the bluetooth calling was working for the following apps.

1. Grid3 (https://thinksmartbox.com/product/grid-3/)
2. CallCenter (https://www.justremotephone.com/) and
3. Thy Phone (https://apps.microsoft.com/detail/thy-phone/9PKT9JS17KXZ?hl=en-us&gl=US)

So few of the apps bluetooth calling is working for 22H2 itself. Maybe there is some issue in our app.

It looks like Thy Phone uses the same PhoneLineTransportDevice API. I'm not sure how they manage to make it work.

[BestOwl]

https://gist.github.com/ADeltaX/285e017a1fefb0723b526246066b9f43

Check this out

[BestOwl]

https://gist.github.com/ADeltaX/285e017a1fefb0723b526246066b9f43

Check this out

Still no luck after trying this.

[Manmenhongyi]

Is there any alternative solution to this API? ? ?

[BestOwl]

In the recent Windows 11 22H2 release Microsoft deprecated a number of APIs used for making phone calls. Previously Thy Phone had used these because they presented a standard Windows dialog for call handling (incoming and outgoing) and the experience was consistent with Your Phone / Phone Link.
https://inthehand.com/2022/10/26/thy-phone-for-windows-11/

Yes, there is a new API, but I can't find any documentation about this new API.

[BestOwl]

Found an undocumented property System.Devices.PhoneLineTransportDevice.Connected
MicrosoftDocs/winrt-api#1715

[Thunder-Ray]

Did you try this? https://learn.microsoft.com/en-us/uwp/api/windows.applicationmodel.activation.phonecallactivatedeventargs?view=winrt-22621

To use this API, your application needs to declare in its Package.appxmanifest file windows.phonecallactivation as a uap13:Extension category. Otherwise, the application won't be able to be activated to facilitate in-app call progress control UI.

[ohernandezv]

PhoneLineTransportDevice.RequestAccessAsync() now will always return "DeniedBySystem", Any solution?

[SibertJabbla]

Has there been any progress on this? I'm having the same issue. I contacted Microsoft to request an access token for the API (as done in a previously posted gist https://gist.github.com/ADeltaX/285e017a1fefb0723b526246066b9f43), their response was "I investigated and this the PhoneLineTransportDevice API was removed from the LAF list and does not require a token.". So the token thing is not relevant anymore.

[SibertJabbla]

I have made some progress regarding this issue:

1. The feature is locked behind LAF (limited access features), but this has been reverse engineerd (see this blog and the previously posted gist)
2. After much, much debugging, I discovered the API searches for the following registry entry:
   Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Bluetooth\Audio\Hfp\HandsFree\BypassRegistration, which doesn't appear to exist. When I created this entry and assigned value 1, in conjunction with unlocking the API, I got actual access to the API. RequestAccessAsync, RegisterApp and Connect all apear to work for me now (on Win11).
   Important to note is that I haven't got any functionality to work yet, but it's something I guess.

One alternative to the API is building a kernel driver which implements HFP. The protocol in itself is not that complicated, but the routing of the audio is. The signaling part of the protocol can be done outside the kernel, but the opening of the audio channels appears to be. I also understood that getting a kernel driver approved for Windows is not easy and may even be expensive.