For those that don't or haven't RTFM, this feature is all explained on page 50 of the manual-Internet Connection Feature. If someone hears this tone and doesn't know how to turn it off, or hasn't had a chance to RTFM, when you press the mic, wait for a count of 2 before saying anything. If the INT function is ON, the first couple of seconds of your voice will not be transmitted. I have used FT8900 mobile and base for 15 years without any problems. However, I did RTFM and tested out the INT function on simplex so I knew how to turn it off if it accidentally got turned on.