Dual SIM Automatic data switching

wizoatk

Settings > Network and Internet > SIMS > Automatic data switching

Use data from either SIM depending on coverage and availability

I'm having trouble finding much detail on how this feature is suppose to work on either Pixel stock or GrapheneOS. I presume it's some sort of magic that's supposed to be better than a simple failover from primary to secondary.

Any documentation or clues available?

Grapi

wizoatk

I believe this is self-explanatory: you designate a primary SIM for mobile data. If it becomes unavailable, such as due to lack of network coverage, the system temporarily switches mobile data to the secondary SIM.

wizoatk

Grapi If it becomes unavailable [...]

It's more nuanced than just switching on complete failure. For example, the phone can and will temporarily switch from a perfectly usable network on the primary SIM to a preferable network on the secondary SIM and when conditions change sufficently it will switch back to a network on the primary SIM.