In slot one, the choice is relatively straightforward. Both Shadow Shock and Nether Blast have no cooldown and hit for a decent amount, although Shadow Shock hits a tad harder up front. Nether Blast deals more damage every time the ability is used, but still caps out lower than Shadow Shock. The one advantage to using Nether Blast is the 100% hit chance versus Shadow Shock's 85%. Shadow Shock is an Undead ability and Nether Blast is Magic, so it may be useful to take your opponent's strengths and weaknesses into consideration here.
Slot 2 offers an interesting choice between Siphon Life and Prismatic Barrier. Siphon Life does a low amount of damage for three rounds, but offers a small heal on top of that. Prismatic Barrier is a very unique ability that blocks two attacks, both allied and enemy -- only Thundertail Flapper, Mr. Bigglesworth, Snowy Panda, Coilfang Stalker, and Spectral Porcupette offer something similar. Used correctly, Prismatic Barrier can be a strong defensive tool. Get your timing wrong and you could end up blocking a useful attack of your own or taking a harmful attack from your enemy you planned on avoiding.
Selecting an ability for slot three largely depends on your overall strategy and how long you think a battle will last. Curse of Doom is a very powerful ability that deals a massive amount of damage after five rounds. So before you choose Curse of Doom, you have to ask yourself whether or not it's worth using. Drain Power does mid-range damage but debuffs the target's damage by 25% and boosts your own damage by 25%. Against a Flying opponent, Drain Power followed by Nether Blast would make a great combo. Heck, Drain Power and Shadow Shock would work decently on any non-Aquatic pet.
Overall, Lesser Voicaller is a solid pet with decent damage dealers and a great defensive ability. Even if you don't run this little one in your lineup, it's just too cute to pass up. Especially if you're a warlock!