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As a PHD student, my final project was to connect mobile devices to VoIP networks. I have found Ozeki VoIP SIP SDK during my research and it was a good starting point for my project. It provided the tools for creating VoIP solution capable for communication on mobile devices. With this SDK and Adobe Flash technology I could develop mobile apps easily and it can be exported to Android and Apple mobiles and tablet PCs. So it was a really useful that I needed to write the code once and I could use it on many devices. As Adobe Air is basically a desktop environment I could also create desktop apps with the same Flash code that I used in Flash webphones and mobile apps.
So I think this SDK will be helpful for you too, google this: voip-sip-sdk.com/p_455-voip-mobile-devices-voip
The problem with the convenience of wireless phones is that they cannot necessarily provide as reliable a source of bandwidth as cables do. If communications is important to your company (it likely is), then you'll want to have a wired connection from your handset to your router up to your ISP. Although wireless IP phones are available, there's no way to guarantee QOS with a wifi connection unless one is willing to spend a considerable amount of money building a mesh network, and even then, the wireless signals can meet with interference from sources as mundane as fluorescent lights or as celestial as sunspots.
You already have the wiring in your office if you have Ethernet network connections for your computers. One of the best things about VoIP phones is they can use the same network as your computers.
If you have no network cabling at all, and never want any, see the other answer. But your quality and security go way up with wired connections.
There are many models of VOIP phones with wireless connectivity. A quick Google search brings up models from Cisco, Polycomm, Unidata, Pirelli and other companies. With a little effort you might be able to locate a handset that works well for your company. But just to be a curmudgeon, I recommend that you ask yourself if there’s anything worse than losing connectivity with your customer when you’re on the phone. Wired handsets may require cabling your office, but that means your phone calls will be more reliable, and there will be fewer chances for problems because desktop wired phones don’t rely on rechargeable batteries as Wi-Fi handsets do.