If you are using the same phones are you had before you migrated to DV , then presumably somewhere you allocate a ringtone to each number in your directory, that will basically be a look up table , the CLI ( calling line info ) comes in , the phone checks what ringtone is allocated to that CLI ( the calling number ) and plays the appropriate ringtone, if you have different phones now you are on BTDV , that facility to assign different ringtone’s may not be a feature of those new phones
