I found the issue was inserting the sim card when the phone was turned on. I turned the phone off, took the sim card in and out again, rebooted and wifi calling appeared - in various places, including swiping from the top. I assume various functions are enabled when network checks are done at start up. I knew i had wifi calling enabled with O2 because i had it on my previous iPhone. Just upgraded to the S10+ and live in an area with horrendous mobile reception.
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