I was facing the same issue! I don’t have my Ears (lol) on me but I’m pretty sure this is how I do it:
Open ChatGPT at least one, and make sure your phone’s screen is facing up (for whatever reason I couldn’t get ChatGPT to answer my questions if I started with my phone facing down). Now you can flip your phone over so it’s laying screen-down and it should do the trick.
PS: remember to manually hang up after the conversation is over, otherwise it keeps running in the background (don’t know for how long lol).
EDIT: Also, for whatever reason ChatGPT voice chat quality seems a bit off when activating it through the Ears! Anyone else noticed it or just me?