Curse was becoming more popular than TS at the time, but Discord offered a better quality of audio and stable connection. That's why my group of friends migrated to Discord around 2016.
TeamSpeak was usually the default option at the time (with some on Mumble). Skype had some presence, but was usually grumbled about beyond 1:1 calls. RaidCall was starting to gain some presence because it was free but otherwise followed TeamSpeak's UX, but was still pretty niche.
Curse Voice was absolutely nowhere to be seen.