The question that you brought up wouldn't address your question because "if" is not one of the coordinating conjunctions, but it is a subordinating conjunction. The words that introduce each are:
Coordinating Conjunctions: and, but, or, yet, for, nor, so (a helpful acronym is FANBOYS)
Subordinating Conjunctions: after, although, as, as if, as long as, as though, because, before,
in order that,
The definition of a subordinating conjunction is:
Subordinating Conjunction (sometimes called a dependent word or subordinator) comes at the beginning of a Subordinate (or Dependent) Clause and establishes the relationship between the dependent clause and the rest of the sentence. It also turns the clause into something that depends on the rest of the sentence for its meaning.
He took to the stage as though he had been preparing for this moment all his life.
Because he loved acting, he refused to give up his dream of being in the movies.
Unless we act now, all is lost.
This article from Purdue on coordinating and subordinating conjunctions explains the usage of the subordinating conjunction as follows:
Notice that when the subordinate clause comes at the beginning, it’s necessary to insert a comma.
From this, the correct punctuation of "You can call me if you need me" is:
You can call me if you need me.
If you were to move the subordinating conjunction to the beginning, however, you would need the comma as follows:
If you need me, you can call me.