To provide a base station apparatus capable of preventing disconnection of a call session caused by a wireless connection state, a wireless terminal apparatus, and a method for retaining the call session.

An SIP server 16 has a timer that counts disconnection timing of an SIP session established between a wireless terminal 12 and the called party terminal 18 after S100-S116. The wireless terminal 12 transmits a re-INVITE message (timer initialization request) for resetting the timer to the initial value to the SIP server 16. A base station 14 that carries out wireless communications with the wireless terminal 12 monitors the wireless connection state between the wireless terminal 12 and the base station 14, and if the wireless connection state becomes a predetermined state (S120), transmits the re-INVITE message to the SIP server 16 (S124) in place of the wireless terminal 12.

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