bredbandtelefoni - change ringing time

The TDC service claims there is a way to change the time it takes until voicemail turns on ... I have not found a way to do that. The phone rings only very shortly - sometimes even jumps directly to the voicemail. Often no chance to reach the station in time. While this happens on the receiving end, for a caller it appears to ring several times - but even this is not very consistent. Back to my question: How can I extend the ringing time ...   tak for hjelp!   Stefan