Peugeot 406: Beeping of radio without correct code
You need to connect the car to diagnostics over OBD2, using Lexia3 cable and pp2000 software. Car maintenance services usually have this diagnostic kit, or you can buy it yourself for ~50-70$, for example, from Gearbest (https://www.gearbest.com/pp2000-_gear/).
However, it is NOT easy to setup Lexia3/pp2000 as you need to install Windows XP and you might need to get and install newer versions of Lexia3/pp2000 software. So, I would recommend doing it yourself only if you have IT knowledge or have experience on diagnostic kits.
Connect the car to pp2000, go to radio module, and change radio VIN code to match the VIN of your car. It fixes the beeping.
I bought this radio from other Peugeot 406. Same radio fits sedans, estates and coupes.
Read more about Lexia3, Lexia, pp2000, Diagbox here: http://blog.obd2diy.fr/2017/06/01/lexia-vs-diagbox-vs-peugeot-planet-pp2000/