Elvox 940A/887B Audio Issue

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