The correct answer is "as soon as he comes in."
Adverb clauses are chains of words that work together as an adverb. They can modify a verb, an adjective, another adverb, or a phrase. They give extra information, such as when, why, where and how. They possess a subject and a predicate, and start with a subordinating conjunction like as, after, and unless.
In this example, the adverb clause modifies the verb call.