Hi bveseli,
Welcome to Apple Support Communities! We appreciate you reaching out about this issue while in a phone call, and would love to help sort it out.
To start, let us know more about what's going on. Does the button flicker in every call? Have you had a chance to restart your iPhone since noticing this behavior?
Do you have any Bluetooth audio devices paired with your iPhone. If so, head into Settings > Bluetooth, disconnect those devices, then place a test call to see if the issue remains.
If the issue persists, let's investigate further. Do you have any AirPlay devices nearby that can be used for audio during a call? If so, go to Settings > Phone > Calls on Other Devices, and turn off the option for Allow Calls on Other Devices, then test again.
We're here for you if you've got any questions.
Have a great day!