In VOY: Unity the crew answer a distress call from a group of ex-Borg. Although they claim to need assistance in freeing themselves from Borg influence, it turns out that they're being deceptive.
In VOY: Nightingale Voyager responds to a distress call from a ship that claims to be transporting medical supplies. In reality it's transporting experimental technology.
In VOY: Revulsion the crew render assistance to a holographic person in distress. He turns out to be psychotically anti-organic and tries to murder Torres.
In VOY: Fury the crew responds to an Ocampan distress call from Kes. Unbeknownst to them, she's gone completely doolally tap and wants to kill them all.
In VOY: Equinox the Voyager answers a Federation distress call from the USS Equinox. Although initially appearing benign, the whole crew turn out to have a very slim grasp on morality, with deception, torture and mass-murder among their various transgressions.
In VOY: Warhead The ship renders assistance to an artificial intelligence housed in a warhead. It commandeers the ship and resists attempts to prevent it from carrying out its mission to destroy an enemy installation. Although it's neither deceptive, nor antagonistic toward the crew (beyond commandeering their vessel for a short time) its actions aren't what you'd describe as friendly.
In VOY: State of Flux the crew of the Voyager go to the aid of a Kazon-Nistrim vessel experimenting (unsuccessfully) with alien tech stolen from Voyager. The sole survivor dies before we can learn anything about him but in general the Nistrim are not a friendly people.