Slight delay in disconnecting my VOIP phone

Hi, after the incoming call has hanged up, I find my VOIP phone will take about 7 to 8 sec before the VOIP will disconnect itself from the call. This is my disconnect tone setting : [email protected], [email protected];4(.75/.75/1+2) from my SPA3102. May i know which setting should i configure so that my VOIP will disconnect itself 2 or 3 sec after the incoming call has hanged up? By thee way, I am using FreePBX 2.8 and Asterisk 1.8.5