Which is going to win between VOIP wireless - 802.11B/G or 1800 mhz Dect?
Quality of Service helps and works to move voice calls to a higher priority than data services. Administration is through a single console as opposed to two consoles required for DECT (although some companies are working to port the administration). Real throughput is quite a bit less than the rating of the network due to overhead, etc. Other considerations are costs (equipment, administration, cabling, training, ongoing support, etc.)
http://www.dect.ch/pdf/dect_positioning_version1.pdf is an excellent article you may want to read. It compares the technologies in its 80 pages and can provide you with some insight.
The biggest thing to consider is saturation and security. 802.11 can be a bear if not properly configured at the onset. This pertains not only to your installation, but that of anyone else in a shared tenant space or office building as you can "accidentally" attach to their devices if they do not have them properly secured and you are not properly configured.
This was first published in September 2003