That is the most common cause yes. I found (in the past) that the microphone is best placed in the headlining edging, somewhere over the driver's door and the speaker either on the dash or run through the car's stereo. This problem does not, of course, exist with the Bluetooth Earing.
The experience was first gained with the incar two way radios we used in the security company I worked for and put to later use with hands free mobile kits before I decided that there was no longer anything so urgent that the caller couldn't leave a message and have me ring them back from my next stop.