Who I have the same questioned this topic

I have looked at other threads and none seem to fix the issue. 

Under settings:  More; I have selected Wifi calling on. 

in phone app wifi calling is turned on, I have selected to receive calls. 

Data roaming on, preferred network type is LTE/CDMA

My wife's iPhone can make wifi calls but has trouble with texting. 

I am using 7.1.2; but i also tried 8.0 DP3 & DP4.

I am using build version NHG470

I am using a Google Wifi mesh network with three nodes; I have tried IPV6 and IPv4 on my wifi network.  

I tried setting a static IP to my phone. 

I can make a call and it will connect with the other person, but the connection is only for a few seconds to a minute.   I can answer a call, but again only for a few seconds to a minute.   Any ideas on how to fix this?