You can decide after how many seconds your voicemail is activated. Your options are 5, 10, 15, 20, 25 and 30 seconds. A few examples:
To activate your voicemail after 30 seconds of ringing, enter (*)*61*+32486191996**30# and press 'call';
To activate your voicemail after 25 seconds of ringing, enter (*)*61*+32486191996**25# and press 'call'; etc.
This call is free of charge in the EU. Outside of the EU, it will be charged at the rate of a normal call in that country.
( * ) means that you have to enter two asterisks for some devices.