This is from ATT Q&A on voicemails: Should be able to set up to 9 rings, or just call ATT and they can adjust your settings.
To change the number of times your phone will ring before voice mail picks up, call toll free 877-345-RING (877-345-7464). When you call the toll-free number, the system will ask you to enter your mailbox number and give you the following ring options:
Ring Cycles
# of Seconds # of Rings1
12 - 18
3
18 - 24
4
24 - 30
5
30 - 36
6
36 - 42
7
42 - 48
8
48 - 54
9