In modern standard English we have an interesting construction whereby a preposition combines with a verb to form a new verb.
Verb + preposition = new verb
In this case, the preposition isn’t acting as a preposition; it’s acting as a kind of enclitic particle attached to the verb, though in written English we do not attach it to the verb, at least not any more. When this construction was new, the preposition was attached to the verb with a hyphen, and you can still see people of a certain age, or those who were taught by those people, writing this construction that way.
Jarvis hung-up the phone.
Examples of verb-particle combinations or phrasal verbs are shut up, f**k off, pick on, hand in, dress down, blow up, think over, and so on.
Polly handed in her test early.
Horace dressed down the tardy employee.
We can tell these from prepositions and from adverbs in a few ways. First, while prepositions and their noun phrases function as a phrase, here we have a verb and a preposition/particle functioning as a phrase.
Horace ran down the road.
Horace ran down his wife’s cousin.
Ivy ran into Elvis downtown.
Ivy ran into the woods.
Second, often verb-particle phrases can be replaced a different, single verb.
Horace criticized his wife’s cousin.
Ivy met Elvis downtown.
Learning to recognize which group of words is acting as a unit – the verb phrase or the prepositional phrase – is a more reliable test, however.