Curse Voice. Razer Comms. TeamSpeak. Ventrillo. Mumble. Skype (if you really have to).
There's so many Voice Chat programs out there already. All Riot really has to do is endorse a couple of them and players will have them readily available if someone invites them to their call/channel/server in Solo Q. And I'm fairly sure most pre-made teams would already be using one of these programs.
The big difference is that these programs wouldn't put more strain on the LoL Servers, they wouldn't require a separate Riot team to update and maintain, and they already have features to Mute/Kick/Ban/Suspend people if necessary. If someone was toxic in LoL Voice Chat there would be a lengthy process to punish said player. If someone is toxic on your Server you can ban or suspend them yourself.