Put the phone in airplane mode. Only turn off airplane mode when you want to check your voicemail or make a call. This will stop all celltower tracking which is the only type of tracking GrapheneOS can't do anything about.
I'm using VoIP with my phone in airplane mode. Calls come in over wifi if the phone is connected and the voip app is active. When I'm not connected, calls are sent to voicemail (in my case are sent to my email account as an mp3 attachment)
My voip provider is very kyc. I'm thinking of switching to JMPChat.