I had the same problem, and I figured out how to fix it on my phone, your problem could be different. Do you have an unlocked phone? Mine is, and I suspect this causes the APN setting to default to basic settings where MMS (pictures messages, multiple recipients) is disabled. Maybe this can happen on branded phones too.
I went to my carrier's site (Ting) and found a page that explains how to set this up. On the phone this is under Settings - Connections - Mobile networks - Access Point Names. Tap "Add" and input the correct settings. Not all settings are needed, just the ones they give you.
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It appears that the issue is resolved.
There may be more than one undelaying cause, but the main one is confusion. It appears that T-Mobile, I assume, decided not to continue using the Wi-Fi calling status icon in Pie, yet some other carriers seem to still be using it. This obfuscated the issue until I discovered another way of determining if Wi-Fi calling is working. In the phone app, select the keypad. The green phone button gets Wi-Fi arches in it if Wi-Fi calling is working. It can take a few seconds before the button shows this.
Things I did to resolve the problem was to clear the cache partition (google this) and the Wi-Fi Calling app cache. I also re-entered the APN settings (Settings-Connections-Mobile networks-Access Point Names) to the ones recommended by my carrier, which had gotten wiped out by the Pie update. This re-enabled MMS, and possibly also Wi-Fi calling, but I am not sure about the latter.
Since our phones are unlocked we get the firmware updates later than branded phones, and this seems like a wise decision by Samsung. We have had no other problems after the Pie update, just the expected unfamiliarity with the new One UI interface.
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Wi-Fi calling was working great on our two S8 phones under Android 8. Last week the phones updated to Pie and Wi-Fi calling stopped working at all. The Wi-Fi calling notification does not turn on, and calls are dropped as cellular reception fades out. As far as I can tell, it is the only feature not working. Phones are unbranded, unlocked US versions model SM-G950U1, no carrier-specific software installed. I've had mine working with Wi-Fi calling for more than a year before the Pie update.
Our provider is Ting which reportedly uses the T-Mobile GSM network.
This is a deal-breaker since we have very poor cellular reception, and we dropped our landline a while ago.
One strange thing is that the Wi-Fi calling on-off switch and the Wi-Fi Preferred switch act like one switch. Flip one switch and the other flips too, in the same direction.
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