Who I have the same questioned this topic

It appears that the wi-fi calling feature that has finally rolled out for iPhones with iOS 9.3 will require some firewall changes for corporate firewalls.

It looks like the feature establishes a per app vpn for the calling feature. IPsec and ike protocols appear to be in use.

I'm hoping to find documentation from Verizon on what ports and addresses to enabled for this feature.

Can anyone point me in the right direction?