Reverse parking buzzer does not beep when shift to “R

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I have recently bought used Lexus IS250 F Sport GSE30R 2013. For some reason, my reverse parking buzzer does not beep when I shift to “R” anymore. I am pretty sure it was there (deep beep sound like di-di-di) when I got the car a week ago but after serviced from Lexus after a few days it disappeared! Contacted them but they said to turn on my “intuitive parking assist” switch. But I want the buzzer to beep always or by default when I reverse my car or *BLEEP* to “R” regardless of turning parking assistance button on/off. I do not need front buzzer all the time. 

My model is equipped with Front and Rear parking sensor and has a dedicated button on my lower right-hand side (below ODO button) to turn “intuitive parking assist” buzzer on/off.When I turn this on a green cone icon comes in my dashboard, then yes, I do get fine beep sound (ti-ti-ti) for both front and rear distance but when I turn this off then I do not get any beep when I shift to “R”. I have spent hours reading the owner’s manual and reviewing the car’s onboard display system and also disconnected battery to see if this resets but so far, no luck. 

Can anyone please advise me how to turn on the reverse parking buzzer without using “intuitive parking assist” switch? Or this is how Lexus parking system works? 

Thank you.