In station config go to feature keys, choose which line key, for function type pick message center, pick extension not in use (example 320)
Go to dial action table 1 (this is usually default) choose what number you want to use, (any except 3) set the action as rec 1, set routing to what you defined in function key (example 320)

The line key you defined as the message center will flash when message is left.

We get old too soon, smart too late