I have the same issue with dropped calls.
Originally I had comcast 5/15 with three devices connected to my router
modem-->router-->arivana/sip phone/wireless router
My SIP phone never drops a call but Airvana does.
Then I switched to fios 5/25
FIOS modem router combo-->arivana/sip phone/wireless router
The wifi on my fios router is off. Again my SIP phone (callcentric) never drops a call
so I doubt it's the ISP.
I upgraded my FIOS to 25/50 in hopes that would solve it, but no luck.
If I power cycle the airvana it's better for a day then back to only making short (<15mim) calls.
I noticed when on Airvana, the signal bars on my phone sometimes go from 5 to zero, thats with me in a fixed location.
I called Sprint and they increased the airvana power to MAX. So when it works is works way out in my yard which is great. But still dropped calls.
I have 4 phones in house and they all drop calls (htc one, iphone 5, iphone 4s, iphone 4).