If you want to be sure you cant be tracked, monitored, spyed on, and calls can't be intersepted:

Don't ever connect it to WiFi and don't insert a sim card.

Graphene or not, your ISP can still share your position or other meta data with government and stuff (in the us they can also be forced to not tell you) - in some countries they legally sell to third party's, in some probably illegaly

Calls are normally not encrypted so the os doesn't matter as much if its the government who can force your ISP or if someone is skilled enough for a Man in the middle attack.

Android is a highly complex system, it will never be 100% safe.

If you just want to decrease spying by companies and less powerful people:

Use neo store or fdroid (no google play or aurora) as all apps there are Foss

Don't install gapps or any other google services/packages

Use shelter for less trusted apps

Use netguard to block apps from accessing the internet

Physically block your cameras

If you want to be absolutely sure no one is recording audio: destroy mics with a needle and connect headset only when you need it

To only use communication apps which are encrypted and you hold the keys should be not needed to be said: matrix, signal, element, xmpp are good, (telegram (normal chats), Facebook, WhatsApp etc is a no go)