No calls using WiFi

I’ve been having this issue for years on my iPhones .  Nothing fixes it .  I’ve had this problem since I have had Comcast’s Xfinity service since 6/2016 .  People stop calling me because it goes directly to voice mail. They will send me a text saying I cannot call you goes straight to voicemail.

Im so frustrated with it and [Edited: "Language"].  I don’t have cell service in my area , when I’ve had other issues Xfinity tech has to leave my area drive up the road to get cell service. So it’s a major problem not getting calls. 
if they cannot fix the problem I’m looking to go Starlink .  This is total [Edited: "Language"] ! A known issue for a maybe 1% of us but know solution.