Glad I don't have an iPhone then
I can see my phone pinging the VPN every 20 seconds in the router diagnostics as a keep alive, but this task is offloaded to the network/Wi-Fi chip and doesn't drain the phone, i.e. it doesn't wake up the main CPU to do this task. This is with a Pixel 2 XL.
I've gone all day with less than a few percent battery power lost with it on Wi-Fi calling if I've not used the phone for anything. If working from home all week, where often I don't need my phone as on the PC, I can go 5 or 6 days between charges with Wi-Fi calling on.
Maybe its the Wi-Fi point you are using that doesn't support power saving, that can easily cause a big power drain as the Wi-Fi is constantly powered up and not allowed to disconnect and sleep between data exchanges.