voice calls from a mobile phone uses so little bandwidth its barely a blip on your data usage with your ISP.
ive used both generations of this model. i had the samsung unit when it was initially launched a couple years ago and then i picked this latest one up over a year ago when it launched. never had any issues with drop calls, sms messages getting delayed, GPS issues,, nothing.. so i can assure you putting the femtocell behind the router is a safe setup and the most recommended. the QoS in the box does NOTHING for the mobile calls..
you are free, however, to locate the femtocell however you want it. if you want sprint to control your QoS then let the airvana be the first in line after the modem. choice is yours.