As already suggested, the easiest solution is a DECT phone plugged into ATA
which in turn plugs into an ethernet port of a router. Router with built in
VoIP ports (Draytek, Zyxel, Zoom) is even better as it can set priority for
voice traffic - aka QoS.
Finally you can buy a WiFi VoIP phone such as Zyxel 2000W, which will even
work with Hot Spots in public places, but they are still darn expensive
~£150 (was £200).
Some day soon (or so they say...) there will be GSM mobile phones with WiFi
VoIP built in,
but I am not holding my breath.

I use Voipbuster and Xten softphones over WiFi and have QoS with Draytek
2600VG router,
will switch to DECT as soon as I find suitable handset / deal.
Regards,
Martin