VOIP does not necessarily need to be run through FMOD, you can use a 3rd party package such as Normcore or Vivox. Such packages would however rely on Unity’s internal audio system, so you would need to re-enable that in the Unity settings, which would also mean you couldn’t ship on PS5 or Xbox.
There have been some good discussions on this topic previously, I recommend you look through this thread: Using FMOD with voice chat utilizing OnAudioFilterRead(), which has lots of code samples and various resolutions to issues encountered along the way.