I tried to install the intercom on an Elvox 940A and Elvox 887B system. I find the same problem on both. The app recognises the call signal correctly and opens the door. The problem occurs if I try to answer the intercom. At that point, a deafening whistling sound is emitted from the apartment building’s intercom.
The service department has given me several attempts, but none have been successful.
I read this topic about someone with the same problem that was solved
Hi @abyss, Welcome to the Ring Community!!!
I’m sorry for the inconvenient, usually issue like this can be solved adjusting the mic and the audio setting by the app.
Thanks for providing the pictures. This helps already our team.
Our expert are taking a closer look at this for you and we will get back to you as soon as we know more.
With support team I already tried this solution but doesen’t work for me 
Hi @abyss,
I am so thankful for your patience.
I have sent you a private message with your personal reference ID.
Please contact our Ring Intercom customer support by emailing or calling our team, and we will be more than happy to take a look at this with you.
You can find our support number here: Ring Intercom Support .
We look forward to hearing from you.