WiFi calling and Iphone SE battery life

This is a report rather than a question. I have two iPhone SE (2020) phones and two SIM cards. One of the SIM cards (SIM1) uses WiFi calling as we are in a poor reception area, the other (SIM2) does not. Both SIMs connect to the same mast and use the same carrier network. Recently I upgraded both phones to iOS 17.0.3.

Experiment: I carried out the following experiment: (a) insert one of the SIMs in each phone, charge the battery fully and leave it on standby until the battery runs down.; (b) swap the SIMs over and repeat. I have repeated this for 3 occasions for each phone.

Result: the phone with SIM1 (WiFi calling) has a battery life of 8-9 hours at best, whereas the phone with SIM2 (no WiFi calling) has a battery life of 24-28 hours or nearly three times as long. There is no practical difference from messages (both phones receive them), phone calls (because I carry out the test overnight), etc

Conclusion: leaving WiFi calling on has a drastic impact on battery life under the current version of iOS 17 on this model of iPhone. I have no idea why this would be the case, but for me the difference is clearly large and significant. [By the way I am a statistician. Of course a larger sample would be more powerful but who can waste time when the difference is as large as this.]

Background: the reported battery capacity of both iPhones is 85-86%. Both phones operate in Light mode with a low screen brightness. This shouldn't matter as the SIMs are being compared in the same phone. Apart from WiFi calling I can see no reason why changing the SIM in a phone should affect the battery drain. Both phones have a WiFi connection to the same WiFi network which uses high performance UniFi access points and operates under WiFi6. The phones are being used in the UK but I can't see why that would be relevant to battery drain and phone technology.

Finally, a battery life of only 9 hours is pretty useless, but at 85+% capacity it should be unnecessary to replace the battery. I can't do without WiFi calling since our signal (4G) is poor & erratic. Since WiFi calling is basically Voice over IP, the implication is that there is some drastic difference in power consumption for VoLTE (standard 4G calls) and VoIP calls. What is going on?