Here's why:
* Present Participle: "Calling" is the present participle form of the verb "to call". It can be used in continuous tenses (e.g., "I am calling you.")
* Gerund: "Calling" can also act as a gerund, which is a verb used as a noun. (e.g., "Calling you was my first priority.")
However, "calling" can also be a noun. For example, "This is my calling" refers to a person's vocation or purpose in life.
So, the answer depends on the context. You need to look at how the word is used in a sentence to determine its function.