In the past I tried some wifi phones, none worked well, now we are using a dect sip phone from Siemens, the gigaset c450 ip and it’s working very well.
The phones connect wirelessly to their dect base station, the base station is connected to a lan switch with an rj45 cable, in this same switch Asterisk is connected too with an rj45 cable.
I setup just one of this phones some months ago, for one of my coworkers and since then I never touched it anymore, the setup was very simple, in just 5 minutes the phone was working, if you need I’ll send you more informations about the configuration.