BenQ M555C - Manual (Page 44)

28 Using your phone
Set group
Adds the phone book contact to a preferred caller group list.
/ What is a caller group?
A caller group is a group of callers who share the same incoming-call
settings, i.e. ring tone, image, backlight color, etc. When a member of a
particular caller group phones you, your phone makes the group's ring tone
and displays the image and backlight color assigned to the group, so you can
identify immediately which group the caller belongs to before you answer
the call.
When you save a new contact to your phone book you can choose a
particular caller group for the contact.