Hi! i am currently recording a song in GarageBand, and I want to add the telephone vocal effect to my vocal line, the only issue is that it keeps saving the current vocal line and won’t let me change any effects on it. How do I get it to stop doing that, so I can add fx?

Of course plug-in FX should be saved just as you left them.. Maybe the Track is locked—if you don't see Track Lock buttons for each track, hit Option-T to add them to the track headers, and check that the lock isn't on for the track you're trying to add & save effects on..