Since Pixel phones have perhaps 1% of the market I'd guess the word "never" may feature in the answer. Particularly because with integration of Virgin Media Mobile into O2 a near certain outcome in the next couple of years, VM won't be in a hurry to invest in their current mobile capabilities. As and when the O2 integration happens, you'll be no better off because they don't appear to support wifi calling on Pixel devices either.
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