Attended Transfert there is no sound of notification

we use microsip as softphone
Features.conf: atxfer =>**

My issue is
let me show you the scenario

CalleeA call and operator B answer the call
operator B compose **10005 to call operator C
The issue is operator B does not hear any notification sound and then operator B dont know if the call of operator C is ringing or operator C is picking the call or not.
Does someone have an idea how to correct this issue

Thanks a lot

Are you able to try a different soft phone ? Or a desk phone ?

I use 3cxPhone and it is ok