Firewall configuration and AT&T iPhone Wifi calling

My iPhone has wifi calling enabled--when the cellular signal is low, it can make and receive phone calls using the local wireless network.  It works fine pretty much everywhere I've tried it except my house, on my X4 R7500v2.  I assumed it was a firewall setting but couldn't find it.  As an IT guy, I figured I must have just overlooked it, but after Googling, I found that the firewall configuration is completely hidden.  http://kb.netgear.com/app/answers/detail/a_id/25491/~/r7500-faqs

So, I found this forum and the release of openWRT for my router.  The feature list for this release of open WRT makes no mention of the firewall at all, so I was wondering if anyone can tell me if it will allow the firewall to be reconfigured or disabled.  I am NATted behind the ISP's modem/router which is also firewalled, so disabling the firewall on the X7500 would not present a security risk.  Just to make sure it wasn't the ISP's modem/router that was blocking the wifi calls, I enabled its wireless and wifi calling worked fine through it.

Thanks in advance!