While trying to post the icon (which I am unable to figure out) I actually came across the following explanation for the symbol:
"Phone Headset with Wi-Fi Signal Icon
If you have come across this icon, do not panic, you are not in an active call and your monthly telephone payment is not going to be sky high. This is the new feature that is used when the phone has set up a Wi-Fi call. This is a relatively new extra that allows users to make phone calls via Wi-Fi instead when the conventional mobile signal is weak."
So this is a new feature that just seems to have coincided with the occasional warning of international calls. Perhaps I don't actually have a problem, but it still does seem strange that I am getting warnings about international calls.
Thank you for your interest.