Had this problem even though my phone did not support Wi-fi calling, it was activated on my account. After reading about this problem I went into my local store and after the usual diagnostics, got the assistant to switch off Wi-fi calling, which the assistant was surprised to see was activated on my account. This restored diverts to voicemail. I thanked the assistant and asked if she knew what the problem had been to which she replied “I have no idea”. I appreciate she did her best, but without reading about this problem on the Community, I would have had no idea myself how to resolve the issue.
