I've use three different (pure) WiFi SIP phones (all of which have been discontinued now) I have found that all, have had battery issues (short life span). One combo phone GSM + VoIP had no STUN and so could no be used outside the local LAN. In my experience, they are convenient if you want to make a call when travelling and have no mobile phone access or costs are very high. Some sets, on some hotspots will loose registration after some time and so can't be counted on, in the long term, for receiving calls. In the past, you could pick up some of these sets for about $100 US, and there are still a limited number of consumer quality sets around that price. The majority of "office"grade sets start about 3 times that price.
As abc123 has said, an ATA plus a DECT set would tend to be more economical, not only the initial outlay, but there would also be a big savings in the long run replacing any lost or damaged handsets. The whole wireless component (set and base) could be replaced for the cost of a new WiFi set battery , in many cases. While the range of DECT would not be as great as it would in a site with multiple WiFi access points, some DECT systems now come with repeaters.
