Odd wifi calling issue?

Hi @BobbyT 
Hmmmm  It does appear that the phone's software are similar BUT it does make me wonder when you have the exact same phones your Software states '0017  R58N35DSL5F' and your wifes states '0011  R58N72YGRLP ' and if that is where the problem lies  but I don't think so because the software, on both, is 'unbranded' and the updates should happen and not be restricted.
If your wife has the capability to use the 'LTE' (VoLTE) network to make/receive calls then WiFi Calling 'should' work as the software enables use of both.
I know these are probably a stupid questions (and if there is a stupid question to ask I will ask it ) but:-
1) Does the phone have the 'Wi-Fi Calling' app installed and is it Enabled? (under Settings search for Wi-Fi Calling as mine doesn't show up under 'Apps' )
2) Have you tried swapping the SIMS to see if that is the problem?
3) Have you tried manually updating the software to ensure the most current version is on the handset?
4) When you swipe down from the top of the screen (you may have to swipe down twice depending on the organisation) does it show the WiFi Calling icon and is it on?
5) Have you tried.... 
Step 1. Open the Phone app and then tap the More options icon (the three vertical dots).
Step 2. Select Settings.
Step 3. Find and tap the switch to enable Wi-Fi calling.

Other than that ....... *shrug* 
I hope you find the answer but if not Google is your friend (other search engines are available )

Good luck and if you think I can help you further then please let me know!! x x