The MM entry for a Lich mentions destroying a phylactery is a difficult endeavor but the only way to prevent a Lich from rising again and again.
Does destroying the phylactery destroy the soul within? If not, is there existing lore explaining what happens to a Lich's soul when its phylactery is destroyed?
Lich phylacteries in general can permanently destroy or consume souls fed to it by the required ritual, but presumably the Lich's soul is not one of them so long as the phylactery is intact.
D&D 5e answers preferred, but mechanics and lore from other related systems will be noted for guidelines if nothing in 5e exists on the topic.