Shorten the number of rings before voicemail

I would like to change the number of rings before my calls go to voicemail. Currently it rings for approximately 30 seconds before going to voicemail and I would like to change it to 15 seconds. I have read a lot of answers on the internet saying to simply call *#61# and record the number shown for the voice call forwarding when unanswered. The problem I am having is that all the shown features are shown to be disabled so no number appears. How do I enable this feature? My carrier is T-Mobile.