This combo results in no special effect.

In order for the hail of thorns spell to activate, you have to hit with a ranged weapon attack.

The next time you hit a creature with a ranged weapon attack before the spell ends,...

When you use conjure volley, you are no longernot making a ranged weapon attack. Instead you are creating a spell effect that forces a saving throw.

Each creature in a 40-foot-radius, 20-foot-high cylinder centered on the point must make a Dexterity save. A creature takes 8d8 damage on a failed save, or half damage on a success

For it to be a ranged weapon attack, it would have to say so. See the booming blade spell for an example of how this would be worded:

As part of the action used to cast this spell, you must make a melee attack with a weapon

What happens?

Conjure volley takes effect as normal duplicating your piece ifof ammunition. As long as you keep concentrating on hail of thorns, your next ranged weapon attack during the duration will trigger the spell effect.

Hail of thorns targets Self not the ammunition so using the ammunition for conjure volley doesn't impact the hail of thorns spell at all.