Raw presence data from a plurality of presence data sources is collected and processed into unified presence data. Unified presence data over a period of time before a first time is stored in a historical presence data archive. A probability of a presence variable having a specific value is generated based on unified presence data collected and processed at a second time on or after the first time, the historical presence data archive, and user profile data. Presence information is generated based on the probability and the user profile data. Presence data sources include both Session Initiation Protocol based and non Session Initiation Protocol based presence data sources. In an embodiment, non Session Initiation Protocol based unified presence data is transformed into Session Initiation Protocol based unified presence data.