If you've got T-Mobile as your wireless provider, this little tip could come in handy.
If you want to increase (or decrease) the number of rings before an incoming call goes to voicemail, from your mobile phone dial **61*(your phone number)**15#.
Where "(your phone number)" is your 11-digit phone number (including 1 and the area code), and "15" is the number of seconds (from 5 to 30 in 5 second increments) that you want your phone to ring before going to voicemail. (The default seems to be 10 seconds.)
You can thank me later. ;)
(Hat tip to Bradley!)