Voicemail has gone down to about 5 seconds how can I increase back to 20.

[Doctorwho7]

My phone has suddenly droipped to about 5 seconds before voicemail cuts in. How can I increase this to 20 seconds?

[jonsie]

To extend O2 ring time, dial *61*901*11*30# (replace 30 with 15, 20, 25, or 30 seconds) and press Call; this sets it to ring up to 30 seconds before voicemail, but you may need to disable WiFi Calling or check phone-specific settings if the code fails. 

Or you can set the length of ringtone in My O2

If you have an iPhone this may not work