UWP: How to send USSD request. (Get exception, when call SendMessageAndGetReplyAsync) - win-universal-app

Question

please help me with sending ussd request.
IReadOnlyList<string> networkAccIds = MobileBroadbandAccount.AvailableNetworkAccountIds;
if (networkAccIds.Count == 0)
{
return;
}
string networkAccountId = networkAccIds[0];
UssdSession session = UssdSession.CreateFromNetworkAccountId(networkAccIds[0]);
2 sim card installed in my phone. But networkAccIds.Count always zero. What's my mistake?
update 1:
Thanks Jerry Li. I got the network identifiers. I have a new problem. When I do:
UssdReply reply = await session.SendMessageAndGetReplyAsync(new UssdMessage("*100#"));
I get exception:
Unexpected exception occured: System.Runtime.InteropServices.COMException (0x80070032): The request is not supported.
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at uwp_1.MainPage.<GetBalance>d__1.MoveNext()
Why do I get exception?

Answer 1

2 sim card installed in my phone. But networkAccIds.Count always zero. What's my mistake?
You are accessing privileged APIs (UssdSession and MobileBroadbandAccount) used by mobile broadband operators. It requires a special-use capability to be declared in package manifest:
<Capabilities>
<r:Capability Name="cellularDeviceIdentity"/>
<r:Capability Name="cellularDeviceControl"/>
<r:Capability Name="cellularMessaging"/>
</Capabilities>
Please make sure you have declared the corresponding special-use capabilities in package manifest. And the Mobile broadband sample which shows how to use the Mobile Broadband Networking API is for your reference.
In addition:
Your app will work and you can test it on your phone, but it will not pass WACK (Windows App Certification Kit) because it uses a special-use capability. This kind of apps can be used in Company app(Don’t need to publish to Windows Store), MS internal or OEM/partner.