Call Center does not hang up

Hi

since i did an upgrade from fusion Ver. 4.2 FS ver 1.6 to Fusion 4.4 and FS 1.8 when a call comes into the call center and the caller hangs up it will not disconnect the call on the fusion end untill they hang up the call

in callcenter many users have a headset so they are not used to hang up the call

any idea?