Plese try using IsDedicatedServer with a Branch before adding VOIPTalker in a Pawn class.
In PlayerController or PlayerState shouldn’t be a problem. But if you are spawning Pawn class on GameMode (dedicated server’s main class), than server owns it. I am not talking about Possesing, talking about Owning it.
I didn’t try on Dedicated Server but I can use VOIP on Listen Server model.
Check Cedric (eXi)'s Network Compendium for more info about Server-Client-Owning-Possessing