The “rephone” specifications hint that its hardware seems to be identical to the Volla Phone 22 specifications and the specifications of the Gigaset GS5. I.e., they likely are all are a Gigaset GS5 in terms of the hardware, but Volla offers it with “alternative” operating systems pre-installed: “Volla OS”, which is a Google-free / AOSP-based Android (likely based on LineageOS), and Ubuntu Touch / UBports. Because the “rephone” seems to run a “stock” Android by Google (there is no indication of any “tweaks” to the OS stack being made), it likely is identical to the Gigaset GS5.
Unfortunately the Gigaset GS5 / Volla Phone 22 / rephone uses a Mediatec SoC (Helio G85), which makes porting SailfishOS a bit harder, but so does the original Volla Phone (based on the Gigaset GS290) and the PlanetCom Gemini PDA, to which SailfishOS has been ported successfully (albeit with a few caveats compared to the Sony Xperias, of which the officially and community supported models all use Qualcomm SoCs).