UniData WPU-7800 is a Wi-Fi SIp enabled cordless phone.
It works reasonably well, but as others have pointed out, DECT is a more robust solution. It is not the easiest thing to provision, the instructions are also somewhat lacking and finding the latest firmware was painful, but at the end of the day it does work.
However, you will need a very robust and stable Wi-Fi network to get the best out of the device. Roaming, if needed, will likely prove to be less than hoped for as has been suggested by others above and for the right reasons.
I use it remotely at the house from the 3CX install at the office so I can sit in the easy chair during the evening and still handle calls while watching TV....if I want. It works pretty well with my single AP.
I only got the device to satisfy my own curiosity and I found one at ABP Tech that had been returned or was a demo and had been steeply discounted. But as I indicated earlier, the DECT handhelds I have do indeed get far more range/coverage than the Wi-Fi and this is largely due to the frequencies in use, obstructions and other issues that affect Wi-Fi more so.