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Project Fi lists the Pixel, Nexus 5X and Nexus 6P as the only compatible phones with their auto network-switching system, and the FAQ and many reviews state that these phones have special radios:

These devices are the first smartphones that support our network of networks. They work with the Project Fi SIM card, which supports multiple cellular networks, and have a state-of-the-art cellular radio tuned to work across network types.

However, in the iFixit teardowns they don't have any unique hardware. iFixit says that the Nexus devices use the Qualcomm WTR3925 LTE Transceiver, same as several other phones, while the Pixel uses WTR3925 as well as the Qualcomm WTR4905 LTE RF transceiver. All of these radios are used in several other phones, so why don't those devices support carrier aggregation?

I found this Qualcomm article from 2014 talking about carrier aggregation radios and they appear to be widespread:

https://www.qualcomm.com/news/releases/2014/09/10/qualcomm-extends-benefits-lte-and-carrier-aggregation-entry-level

Can the hardware be enabled in software in other devices?

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FWIR, the Project Fi SIM contains the two separate carrier profiles (separate USIM applications with their own IMSIs that obtain the lease for one carrier or another) which are selected by some undisclosed means, as noted here (there is some theorization that this could be done by software on the baseband processor or possibly the SIM card itself, and as likely as not actually a combination of those things together with the Project Fi Tycho App.) The Active carrier USIM can be manually switched using proprietary dialer codes, but at the end of the day you have something in the form of software making a selection in the same way you might find on a dual SIM device (which is not a new concept).

The WTR3925 LTE Transciever is certainly key to being able to handle Carrier Aggregation but since the signal gets processed by the baseband and the actual selection of one carrier over another is being handled by software, it would be easy enough to assume that the unique thing about the Google Devices has more to do with proprietary code than anything else.

As far as enabling the ability to use a project fi sim in another device, it has been reported that while it will work with similar hardware profiles in terms of recognizing the SIM card, it still will only recognize the currently active carrier profile and as the specifics of the switching mechanism is not openly documented at the moment it would have to be switched manually or by attempting to implement a unique selection algorithm written to the spec of the PFi SIM.

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