This looks aftermarket, the wire would be tucked along the head liner edge. Look under the dash , kick panel for wires / evidence of an aftermarket phone.

Some MB had an optional built in phone, the handset was in the console and the radio portion in the trunk sharing the regular radio antenna through a splitter. I think the hands free function piped sound through the radio speakers ( LF only ? ) and the mic was a small bubble top left of the windshield.

This is a prime example of how a shop can get blamed for a dent , scratch , cracked part that was there when the car arrived. " I've had the car 18 years and it was not there when I left it at your shop. "