A Microsoft framework for building cross-platform mobile apps using .NET and C# with native performance and user interfaces.
Repeating the same question isn't going to get you a different answer.
We cannot help you with Android telephony issues on Q&A. Leaving out half of your problem description--that it is specifically about calling AudioManager from within telephony events--will not change this.
If you want Microsoft's help with your problem with interactions between Android's telephony stack and its audio stack, you will need to open a support case. It is beyond the scope of what we can help with in Q&A.
Create a new Xamarin Support Request
Select Developer Tools / Xamarin / Class Libraries and Runtime / Android Platform Libraries
If you open a support case we can help you narrow down the issue, but the problem appears to be with Android API and not with Xamarin API. If there is a bug here it would need to be addressed by Android and not by Microsoft.
I'm not sure who you talked to, but it wasn't anybody in Xamarin support. I suspect you posted a development question on Microsoft Answers, and they forwarded you to Q&A because Answers doesn't cover technical questions. They were not addressing your problem specifically.
If you open a case through the link I provided and with that path it will go to the Xamarin support team, and that team will be able to investigate the problem. Again though, it sounds like something that would need to be addressed at the Android level not in Xamarin.
--Rob