I am a long time HTC user, and still own an HTC 10 (a bit old for today’s’ standards). Upgraded to Oreo earlier this year, and lost a very nice feature; Wi-Fi Calling.
Wi-Fi calling helps on lowering costs when traveling overseas, as while on Wi-Fi, any calls to the US or received, do not incur any additional costs. It also allows to send/receive text messages when there is no service due to location (like inside basements).
I searched trying to find out how to solve the issue and found no answers. I had the option available and I ensured it was enabled, but had no luck. I also checked for power settings on Wi-Fi Calling to ensure that they are not optimized, but had no different experience.
A couple of days ago I decided to try a SIM card from a different provider. Used it for a few minutes, and then swapped it with my T-Mobile SIM. I realized that the Wi-FI calling icon fired up. I thought it was a glitch, but after 2 days of normal operation (e.g. going out and using LTE, then back to Wi-Fi), I still see the Wi-Fi calling service active.
Seems that the solution is to switch to a SIM card from a different operator, using it for a few minutes, and then inserting the T-Mobile SIM back. Of course, turning off the smartphone before ejecting and inserting the SIM cards.
Wi-Fi Calling is back!