"Given that two different profiles exist on the Fi SIM, the Fi software must have the capability to switch between them. SIM cards are actually little computers, so by developing an application that runs on the SIM card Google could trigger a switch based on any of the information the SIM has access to – the network it is registered on, the receipt of a trigger SMS, or something else. My guess is that the SIM card co…
Standard smartcards (including SIMs) have a select command[0], which is used to select applications. If there are two USIM apps[1] (what normal LTE SIMs have one of), they can both be listed in the directory file, and the phone can select either based on whatever it likes. I'm not sure what a plain phone would do with a dual-USIM card (search the app directory for a USIM app and use the first or last? Ask the user? Choose based on the network ID as encoded in the IMSI? Other??)
[0] The relevant 3GPP UMTS/LTE spec, http://www.3gpp.org/DynaReport/31101.htm , defers to an older ETSI spec, http://www.etsi.org/deliver/etsi_ts/102200_102299/102221/08.... for the actual command definitions.
[1] The USIM app, defined in http://www.3gpp.org/DynaReport/31102.htm , contains information like the IMSI (readable), perhaps the phone number (r/w), and the secret shared with the network (inaccessible, but the "authenticate" command can use it to generate a set of session keys that are accessible to the phone).
> More likely is that the SIM card just holds a few different profiles and custom software that runs on the baseband processor watches the strength of both networks and sends out of band messages back home to tell recycled Google Voice infrastructure how to find the subscriber.
Or Google provides a P-GW (as shown in https://en.wikipedia.org/wiki/System_Architecture_Evolution#... ) that both networks use; then the mobile could keep one IP address and the voice services wouldn't have to track anything. The SWn interface shown on that diagram is basically a VPN.
