so you are not getting your unit for free? i pay no taxes or anything on the unit.
the documentation is for VOIP service that sprint DOES NOT OFFER. you are not using a VOIP service with the unit.. you are letting sprint route their calls and data through your comcast network. you are helping sprint by offloading your call traffic across your comcast cable.
since the airvana tech support cant troubleshoot other equipment thats connected to it, they recommend you put it before other devices. thats why. its not to give you better call quality, i have been using this service since at least 2008 and i can speak on experience that putting the airvana/airave unite behind the router will work best for the end user who only needs voice calls from sprint on their cellular phones.
all airvana/airave units have a number, thats how sprint sets up accounts. you cant use that number to make any calls to the device. when i had a mobile hotspot on my account, it had a number associated with it. you cant use it, its just for account purposes.
again, if you love your setup, thats awesome. I am just giving you my personal experience.
enjoy your service...