When you say "extend the ringtone" I assume you hear it ringing but it goes to voicemail before you can answer it. If so, that rules out several possible problems.
I'm not certain but I think for most people, this is a setting you change through your carrier, not the phone.
There is supposedly a standard dialling sequence for mobile systems that, hopefully, your network implements. This page offers the generic and some specific codes. GSM (Global System Mobile from the French) is the standards body.
I haven't tried them but I doubt they'll do any harm if they don't work.