Wifi Calling

I'm using an iPhone 6 with latest version of IOS9 & and my wife a 6+ with same IOS.

Due to being in bad ATT coverage area we have wifi calling enabled, which works great at home and also on some other wifi networks.

However we are noticing there are also some wifi networks that wifi calling isn't working, when connected to the network.

Since it works at home where our main problem is I haven't reconfigured anything. I've tried rebooting both phones, but no change.

Anyone know what would cause this? Is it the phone or the wifi network we are trying to connect wifi calling to?