Presence User Agent (PUA) provides SIP-based presence and instant messaging services. PUA server converts protocols for the legacy terminals that do not support SIP protocol.

PUA communicates with IM-PS server through CSCF in the IM network, and communicates with handsets that do not support SIP protocol. Handsets without SIP protocol that are not located in the IM network download the application run on VM and communicate with PUA via TCP/IP.