Try this:

where XXXXXXXXXX is the ten-digit voicemail system number for your area and
tt is the time in seconds you want the phone to ring for. Valid numbers for
tt are 5, 10, 15, 20, 25 and 30. Since each ring is approx. 5 seconds
including the blank space between them, that means 30 seconds should give
you about 6 rings and that's the maximum you can do.

For instance, to set the longest ring delay for the NYC market you would
dial:

*61*+19084630020*11*30#

After pressing the "send/call" button you should get some sort of
confirmation message.

Note: If you don't know what the voicemail system number for your area is,
dial *#61# on your phone.