I was pulling into my garage while still on a phone call. Usually the alerts go off as I approach the far wall. However, this time, the alerts were silent. I tried again with the same result. This did not happen with my previous E63S under the same circumstances. I sense a programming error. I'll report this to MB technical support LE. In the meantime, I suggest getting off the phone while parking!

Is it just that the audio is muted or are there no visual indicators as well?

I don't know if the visual indicators were absent as the indication is only in a corner of the screen and just a squiggly line turning yellow to red. I would have thought that the alerts would have overridden the audio of the call. I have now changed the alert sounds to not turn down to see if that changes the behavior. I'll post back.

Update:
I changed the audio alerts to not fade out and the issue has not reappeared. No idea why the initial alerts don't sound when set the other way, but I'm not going to experiment further.