I checked the Legend's Feature Reference Guide and this is what I found on page 684/1068:
Distinctive Ringing:
Distinctive ringing allows users to identify the type or origin of an incoming call. The system identifies calls with the distinctive ringing patterns listed in Table 40.
These patterns cannot be changed.
There are 8 different cadences to choose from, but the patterns are fixed.
If it ain't broke, I haven't fixed it yet.