2.2.2. In adverbial clauses and phrases

If a dependent clause is restrictive (omission will affect the meaning of the main clause), don't set it off with commas. If it is nonrestrictive (omission will not affect the main clause), set it off with commas:

I'll go to lunch with you if we can get pizza.
I don't want to go out for pizza, because I had pizza yesterday.

If a dependent clause comes BEFORE the main clause, use a comma whether the clause is restrictive or not:

If we get pizza, I'll go to lunch with you.
When I heard the voice on the other end of the line, I was quite surprised.