Unlicensed Mobile Access, or UMA for short, is basically GSM and GPRS over Wi-Fi. Used by a number of network operators and supported by some phones from Nokia, RIM and others, it's an interesting technology and the basis for VoLGA, a voice over IP solution for LTE that I am quite passionate about. Recently, I stumbled over this website which explains how to configure Wi-Fi Access Points that run the open DD-WRT operating system for the use with UMA phones. Some tips and tricks are given but basically the message is that the Wi-Fi Quality of Service Extensions (WMM) have to be turned on. That also answers one of the questions I had as to whether UMA in practice only runs with access points offered by network operators. So if you are into the details the link might provide some interesting information for you, too.