Using mobile as wireless landline handset

I have a voip requirement that is almost the reverse of 'normal' voip pattern of use.

I would like to use my mobile to answer (and sometimes to make) landline calls. This would saves me carrying a mobile and DECT handset around at home. This should be possible through:
phone line <--> gateway <-voip-> mobile with SIP client

The gateway needs an FXO socket, LAN connection (probably wired, possibly wireless) and a very basic SIP server. Basically, the gateway is pretty much like a DECT base station, but with SIP/IP rather than DECT/wireless. Indeed, the gateway could use some alternative voip protocol that could be simpler than (stripped down) SIP, but the drawbacks seem much greater than the savings.

I thought I could configure a device such as ZYXEL P2602 (L version) or Cisco SPA3102 to do this, but it seems considerable overkill of hardware, and of configuration effort. I tried a cheap P2602 but it seems that it won't work as I needed; unless I have missed something the configuration only allows routing to the FXO from the FXS; no SIP->FXO.

I can see the benefit of all the other features such hardware could provide, but it doesn't really apply to my (simple?) need. I would have thought there could be quite high demand for a simple consumer gateway that would require almost no configuration. Does anyone know if such a simplified PSTN/FXO/SIP/ip gateway exists?

The fritzbox 7390 and 7490 have android and iOS apps that let you do this