There are two Pixel4a models available: The G025J and G025N. The "J " model is geared towards the US market and can/could work on the (decommissioned as of today) Verizon and Sprint 2G/3G CDMA networks. The "N" model has a few more LTE bands but no CDMA/EVDO 3G fallback as it is a pure GSM/UMTS device. Seeing that Nitrokey is based in Europe I assume that the Nitrophone uses the G025N as a base device.