I've seen reports of this problem @ Sprint user forums
It's an Airave, btw, not airWave .
Some have suggested that's a phone setting which has to be changed, such as the one which tells your iPhone to allow the phone to take a call while on a data connection instead of it going to voice mail. Definitely spend some time searching through the following threads, and post your own questions there. I don't have enough personal experience dealing with Sprint issued equipment vs ATT & Verizon, unfortunately.
Definitely push them to issue you a credit for their (poor, lack of) service(!) [One reason why I will never bother with low priced carriers that don't support their users well.]
Personally, I would dump the airave and find something that works more reliably with more carriers/frequencies, such as some of the Wilson Electronics repeaters/amplifiers. You can get on board for $300-400, and they use external antennas to bring in the signal outside where it can be used everywhere in your home by your cellular phones. Much better long run product if you live slightly out of areas with good cellular reception of any kind. It uses radio waves and does not rely on a weird proprietary product requiring a broadband connection. Think, cabin on the lake, mind you.