Backup calling on Android (WiFi calling over a data SIM)

This doesn't work anymore on Android 14. For whatever reason, it sounds like Google really wants you to buy an iPhone is you need backup calling. This issue "probably" is only for "Warp 5G" (Verizon) but I haven't tried "GSM" (T-Mobile) yet.

~~This post is about using a secondary data-only (e)SIM to make voice over WiFi (VoWiFi) calls on Android.

I have a Pixel 7 phone. It supports dual active SIMs (either SIM+eSIM or eSIM+eSIM). I assumed if I had VoWiFi enabled on USMobile and an eSIM with international data, I'd be able to make calls internationally. I discovered it was not that simple.

To test before my trip, I simulated being out of coverage by telling my primary SIM to use only 3g data, which is no longer available in my area. I enabled the data only eSIM and disabled WiFi. To my surprise, I was not able to make calls or text.

I found some posts that mentioned "backup calling" but I did not see that option in my network settings. I went to look at the carrier config settings in /data/user_de/0/com.android.phone/files and noticed that carrier_cross_sim_ims_available_bool was set to false. Since my phone is rooted, I was able to edit the file and set it to true.

I rebooted. I still couldn't make calls or text.

I then went back into network settings and noticed that a new option appeared. Just below "WiFi Calling" there was now a new option called "Backup Calling". I enabled that, waited a bit and noticed that the signal icons had changed. I was able to make calls and texts. It worked just like WiFi calling!

A few additional observations:

1. You will only see the backup calling option if you have 2 SIM cards enabled. It will disappear if you disable the second SIM. Interestingly, the value is retained so you don't have to keep setting it if you have reason to disable and reenable the data SIM.

2. The edit to the carrier config seems to get reset sometimes. It happened to me twice so far. It may be the monthly firmware updates or it might just be something the carrier pushes out occasionally. This is over the course of several months so it's only a minor inconvenience.

3. I don't know how to fix this for unrooted phones. (Edit: but you might try this.

4. This is for Android users. IPhones seem to support it out of the box.

5. Based on a recent trip with a Samsung S22, it appears that Samsung phones will also use a data eSIM as data for VoWiFi even though they have no backup calling option. I should caution that it wasn't very reliable for us but each phone is different. My Pixel, by contrast, worked well throughout the trip.~~