Odd wifi calling issue?

Hi BobbyT 
This does sound like a Software problem on your wifes phone (although I am not an expert, just someone who faffed around until I found an answer to my own problem which was similar).
If it were down to a problem with the SIM then it wouldn't work in your phone.
Is the Software up to date on your wifes phone? Does the Software Version on the phone have any of the codes for 'named' networks? When you say 'near identical' what do you mean? Could you add both versions to the post so I can see if there is something in it?
The only reason I can think of that the phone isn't showing the WiFi Calling option (and usually includes the use of VoLTE 4G Calling) is the relevant Software update to enable the use of these hasn't happened due to being 'blocked'. Until the relevant Software is on the phone neither of these options will be available and this is what you are seeing.
I think it is just a VERY unfortunate coincidence that this has happened twice to you now as this is a very 'unknown control' that networks still have over customers with regards to mobile phones. When we think of 'unlocked' phones we think networks have no control but how wrong we are.
I don't know what you mean by 'Could it be the router?'  Please elaborate.

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