Incoming calls ring on all lines on phone

When a call comes in, ALL the lights ring on my phone. It’s like I’m winning the jackpot at the casino. This happens on multiple phones (all in the ring group).
How do I fix that?

What phone brand and model ? Probably all the line keys on the phone are registered to the same extension and that is what is causing them to flash all together. When you want only one extension registered on the phone, register it only once, some phones allow multiple SIP extensions to be registered, but if you are going to use only one extension on the phone, you only register that one extension. I’m quite sure the root cause is somehow related to that, but then again I might be wrong.