Most phones let you enable and disable forwarding to voicemail, but few let you specify the delay before voicemail is invoked.
Setting that delay requires special codes to be sent to your network provider instructing the GSM system.
Fortunately most of the world's providers use the same coding method. This app lets you send the codes to increase or decrease the time your phone rings before dropping to voicemail.
This app may or may not work depending on your service provider.
(The app needs phone call permissions so it can transmit the codes.)