Question

In English grammar, why do we use "to" after I talk to someone but not after I telephone someone?

Answer 1

Because of transitive and intransitive verb rules

Explanation:

Here, talk is being used as an intransitive verb which does not need a direct object (a noun following it, "to someone" is a prepositional phrase, not a direct object)
Telephone is a noun/verb, here it is a transitive verb (it needs to be followed by a direct object) and the direct object is someone