Learn Romanian: Vă sun în legătură cu anunțul.

Yes, you could technically say Vă sun despre anunț, but în legătură cu (literally "in connection with") is the standard, natural phrase to use in Romanian for business contexts and phone inquiries. It sounds much more polite and professional.

Does Vă mean I am calling more than one person?

It could, because vă is the object pronoun for both the plural "you" (voi) and the formal singular "you" (dumneavoastră). In the context of answering an ad, you are almost certainly using it as the formal singular "you" to show respect to the stranger who posted it.

Is the person being called a direct or indirect object in this sentence?

It is a direct object. The verb a suna (to call) takes the accusative case in Romanian. While vă looks the same in both dative and accusative forms, it acts as a direct object here. If you were calling a friend informally, you would use the accusative pronoun te and say Te sun.

Why is it anunțul with the definite article instead of just anunț?

In Romanian, many prepositions force the noun to drop its definite article. For example, if you used despre (about), you would say despre anunț to mean "about the ad". However, the preposition cu allows the noun to keep its definite article, so "with the ad" becomes cu anunțul.