#No, this would not work; conjure volley isn't an attack
No, this would not work; conjure volley isn't an attack
Hail of thorns specifies that its effects apply
The next time you hit a creature with a ranged weapon attack
However, conjure volley does not involve a ranged weapon attack. As part of the spell, you do "fire a piece of nonmagical ammunition from a ranged weapon", but that is simply part of what triggers the spell; you are not making a ranged weapon attack against any target. Instead, conjure volley is a spell that forces those in the cylindrical area of effect to make a Dexterity saving throw.
Ensnaring strike wouldn't work with conjure volley for the same reason: conjure volley isn't a weapon attack (or an attack at all).
For a definition of what constitutes an "attack" (and why a spell that only involves the targets making saving throws isn't one), see the "Making an Attack" section of the basic rules:
If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.