BranoI hate VogonsPremium,MVMReviews:
Currently I have PAP2NA (unlocked Vonage) behind USG200 with SIP_ALG off and I don't have any issues with calls not being received or forwarded to voicemail. I get pretty much every single call and the system works.
What bugs me that sometimes (1 of 10) let's say some calls get dropped in the middle or have echo or noise. After reading many various posts on internet I've decided to ditch PAP2NA that hasn't have FW updated for ages and just ordered the new Cisco replacement SPA112 which has apparently re-written firmware, SIPv2 support and overall all components (i.e. codecs) are updated. It's being shipped now and I'll let you know once I get and install it how it works.
About your particular issue. Try following:
1) Disable all NAT aiding features on the adapter and enable SIP_ALG on router. Make sure you pick the correct port.
2) Disable SIP_ALG on router and enable NAT aiding features on your adapter.
Point is don't let both devices try to figure NAT passing at the same time.
P.S: Can't wait for IPv6 to get rid of NAT once and forever!