I don't own an iPhone, but I always had that problem using Whatsapp, Skype or WiFi Call.
If I start the conversation before I leave the house, when outside the house it drops. I stopped using WiFi Call on AT&T.
Maybe it's not the phone but the Network Carrier Att vs Verizon.
I only try to help, I hope someone else has a better experience to share.
Thank you for your feedback. Network connectivity issues will most likely be resolved more effectively by contacting your carrier directly. If issues, persist, however, please send us an error report to help us debug the issue. Please also keep an eye out for any other issues you may encounter. We apologize for the inconvenience and appreciate your contribution to the beta program.
One UI Beta Team.