My company uses Servpac hosted voip. We have Polycom 550/650s on the desks. The hosted service is fully SIP compliant, so any SIP softphone can join the system. I recently installed WEE Phone on my iPhone. With two settings - phone extension and password - my iPhone is now an extension of my office phone. It works well over 3G and supports PUSH, so incoming calls to the office are received on my iPhone in the background. Calls to my clients are identified as coming from my business. It's the ultimate virtual business phone system.