No, "opposite" is not a preposition; it is primarily used as an adjective or a noun. As an adjective, it describes something that is located directly across from or facing another. When used as a noun, it refers to something that is completely different or contrary. In some contexts, it can function similarly to a preposition, but it is not classified as one.
A prepositional phrase is any grouping of words that describes where, how, when, something is done or which one has done an action. "The ball was found under the bed." is an example of a sentence with a prepositional phrase that describes where something is.
Call as a verb:I will call you tomorrow and let you know what time I will be coming.He called her name, but she couldn't hear him.Call as a noun:Lack of phone service prevents me from placing a call.Telemarketing calls are really annoying.Call is not a pronoun, adjective, adverb, or preposition.