Technical problems and I'm being ignored

In Oct '20, Rivonia Car Radio installed an Android Head Unit into my Mercedes. It cost around R13k. Within a year, the system would randomly and frequently loose the Bluetooth functionality, and I would have to repeatedly reset the entire system. Things settled down for a while after an outsourced technician spent over an hour on the system. The problem has recurred (even worse now) a few months ago, and my main complaint is that Shailesh, the manager, does not return my many calls or WhatsApps. I'm now turning to a public forum out of desperation. In addition, since the installation, my calls cannot be heard ie the hands-free is essentially non-functional (and I have to use earpods if I want someone to hear me!). I had taken the vehicle back when the installation was originally done, and they replaced the microphone, which made a tiny, but insufficient, difference. I want this sorted too.