How have they left it? The mobile team are helpful but sometimes you may want to prompt them to check things :smileyhappy:
When you speak to them again you need to get them to check that the IMS/Sim updates for Wi-fi calling and 4G calling were applied to the sim correctly they have the ability to check!! Rule one thing out at a time!
The updates were applied to all sims regardless of handset, so whilst it shows incorrect on the system it should work.
Have you also tried a network reset on the handset?
I would agree it is highly unlikely to be the iOS device, before you call again ensure that the device is on iOS 11.3 and above, and that wifi calling is switched on in the phone menu.
Get them to check the sim has the updates and then I would ask for a replacement sim to be sent.