Just wanted to respond with my wifi experiences. My wife and I both have an iPhone and Android for our mobile phones (doesn't everybody by now?). Because of this, I did not see the reason to buy a dedicated cordless phone.

Instead, I installed Bria on both phones. It works perfectly, and our mobile phones are also our landline phones. Plus, some clever configuration causes our voip line to automagically forward to our mobile number when the phones are not registered (read not at home) to the VoIP server. A single number reaches us via landline or mobile. It is really nice to get the voip sound quality instead of mobile, without telling the caller "let me call you back on the landline - I am at home now".

So wifi can be a great option, especially when one has the desire for a single number to reach you at home or mobile. But you need a good wireless access point for reliable converage of your house. I recommend the Engenius EAP9550.

If you just want a stand-alone cordless phone, then the advantages and integration provided by wifi and SIP on your smartphone are moot. In this case I would suggest DECT.