Ok, I installed my 868L running v1.12 today connected to a SB6180 cable modem(100/3).
I used a iPhone 4S(9.3.5) and a iPhone 6sP(9.3.2) with WiFi calling enabled on the phone. I presume this feature is all thats needed to use WiFi calling.

The 4S was not connected to the 868L wifi.
The 6sP was connected to the 5Ghz wifi on the 868L.
All 4 ALG options here enabled on the 868L under Firewall settings.
Also my ATT microcell was turned OFF for this test.
Made an out going call from the 6sP to the 4S. I heard my voice on the 4S the switched to the 4S. Heard my self on the 6sP.
Made an out going call from the 4S that was not on the 868L wifi network to the 6sP that was one the 868L wifi network and heard voice on both sides.
Not sure otherwise how to make sure that the actual call was made on thru the 868L.
