From Forum Nokia Wiki

Unlicensed Mobile Access (UMA) enables cellular services (GSM voice and GPRS data) over WLAN. UMA technology provides alternative access to GSM and GPRS core network services via IP-based broadband connections.

• Seamless delivery of mobile voice and data services over unlicensed wireless networks.
• Provides the same mobile identity on Cellular RAN and unlicensed wireless networks.
• Seamless transitions (roaming and handover) between Cellular RAN and unlicensed wireless networks.
• Preserves investment in existing/future mobile core network infrastructure

Mobile operator need to support the following elements:

• Access to mobile network and broadband network with WLAN
• UMA Controller
• Set up of billing system for special pricing
• Online tools for service subscription
• Online tools for configuration

User had to take care following items:

• WLAN access point configured in the device for voice and data
• UMA Controller IP address provisioned
• Security gateway address provisioned