Welcome to Android Central forums. I plugged your question into Google's AI and got a reply that this is not a unique condition and offered some advice for each of the causes it named. You can see that list
here.
I found a thread on Reddit asking this same question. The OP of this thread says it's random for them, and not every call but they may be missing the iPhone connection you believe you've identified. On a side note Verizon responds to this topic. You can read this thread
here.
On the Google support forums someone with a Pixel the same as you posted a similar question. The weird thing with this post is that the recommended answer is to call Verizon. In the AI response they said to ask for level 2 service techs, which I've heard recommended in other threads, it's a way to escalate the issue to someone higher in the chain. A lot of times the first customer no service, I mean customer service people you talk to just read from scripts, and only recommend the most basic of advice. Restart the app, restart the phone, factory reset the device. You can see the thread
here.
Then at Verizon with the most recent post about this issue, from September, someone asked this question. They mention iPhone but this user has a Samsung. So it could be the combo of Big Red, Apple, and WiFi-calling...? Once again the best solution from this thread was to call Verizon's Customer service. Anyway, you see this thread
here.
It sounds like you should be talking to Verizon, and escalating the issue by directly asking for level 2 support or above. That has been consistent in all of these posts and threads. Let us know what you find.