I'm not aware of an auto answer phone setting unless you are using a headset or other Bluetooth device?
If so, it is in the settings for the device.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Currently using: Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)
You can try this: Enter the code **61*901*11*30# onto the screen then dial as in making a call, where 30 is the max time in seconds before voice mail kicks in. You can set it to either 5, 10, 15, 20, 25 or 30 seconds.