Extend length of ring on my Nokia 201 to 30 seconds

"Hi,

You enter a code into the phone which allows you to choose the length of time that it rings before it diverts to voicemail. Usually the ring time allowed is from 15 to 30 seconds with increments of 5 seconds. The default time is normally 15 seconds.

Each mobile phone service provider has their own code so you need to go to your mobile phone service provider's website and either contact them and ask for the code or search for it on their site."