Output, postrouting or forward?

If I’m trying to make a Mangle rule to mark packets that are being sent from a VOIP phone on my network to a server on the internet, should I be using the Output, Postrouting, or Forward chains?

I will quote a famous poster who recently replied on a thread…
“The right one.”

as useful as mikrotik

Input/Output rules are to/from the router itself. And generally, prerouting is used if a routing decision is to be made by he mangle rule. Most other rules will use forward.