I also have the same issue, but sporadic. I was thinking it has to do with the area. I can be traveling and on the phone getting the echo. Then as I move the echo disappears.
I travel and was wondering if I can use this device with more than one TV? Example, set up on home tv then go on the road and set it up on my hotel tv.
I first ordered the Active case and holster. Found the same issue and did not like the feel of the case. So I ordered the Surface case and holster. To answer your question the holster is not interchangeable.