Even Hindu mantras don’t necessarily call in Hindu gods. The most basic mantra is the single syllable “Om,” which isn’t the name of a deity per se, although it’s understood to refer to Brahman, the ultimate Reality behind all the deities.
There are many forms of Buddhism. Tibetan Buddhism uses mantras, many of which call on one of the Bodhisattvas. In Tibetan Buddhism the line between Bodhisattvas and gods is thin or nonexistent. The most famous Tibetan mantra, Om mani padme hum, is usually translated as “The jewel in the lotus,” though in looking stuff up (on Wikipedia) in response to your post I found out that some scholars challenge this and argue that “manipadme” is actually the name of a bodhisattva. (Bodhisattvas, in Mahayana Buddhism, are beings who take a vow to bring enlightenment to all beings rather than seeking it for themselves. As I said, in Tibetan Buddhism in particular they are often indistinguishable from Hindu deities.)
So you’re not entirely wrong–certainly some mantras do call on deities/bodhisattvas/buddhas of various kinds.
But basically a mantra is a sacred chant whose sound is, to use a Catholic term, “sacramental.” The prayers of the Rosary are, effectively, mantras.
The wikipedia article on
“mantra” looks pretty good–in fact I confess that I used it in preparing this post!
Edwin