You have 3.3(4), so you have to change the setting globally, and the number of rings will change for all users. It's in CCMAdmin - Service - Service Parameters, select the CallManager service for any of your servers, and change the parameter for Forward No Answer Timer. Ours is set to 20, which is just a little after four rings.
In CCM 4.X, you can set the timer manually on each individual phone, which is nice.
TMH