Backup calling also does not work for me on Graphene OS. I did not flash the stock OS to test it though.
I have Pixel 6a with a pSIM (AT&T mvno) and a secondary eSIM (KeepGo, data only).
I am able to use Shizuku and Pixel IMS to enable wifi calling successfully on my pSIM. When I use Pixel IMS to enable Backup calling on my pSIM, there is no option for backup calling or switching data automatically on the pSIM settings page. On the eSIM settings page, I see a greyed out option saying "phone will automatically switch to this carrier when in range" that is switched on. Underneath this setting, I see "use this network when it has better availability" switched on. When I select a forbidden operator in the pSIM for testing, data successfully switches over to the eSIM but backup calling does not work. Phone calls attempt to connect through the pSIM and get dropped.
Through Pixel IMS, I enabled FORCE_HOME_NETWORK_BOOL so that my eSIM is not roaming, but this did not effect backup calling.
I suspect there is some value in the API that can be overridden to activate backup calling, but I have not found it yet.
Pixel IMS can easily change the values in this class: https://developer.android.com/reference/android/telephony/CarrierConfigManager
Outside of CarrierConfigManager, something in android.telephony API could be responsible but would require messing around with the ADB console.