This was sort of a thing in the mid-2000s, and I’m actually surprised to see it again. At the time I expected home routers to become SIP endpoints (and that came to pass with fiber), but they all expose FXO interfaces (jacks) rather than act as proxies for soft phones, so there’s a missed opportunity there. But the truth of the matter is that just shipping a DECT phone in a bundle is much easier for the carrier to troubleshoot.