Oracle indexes: what proportion of a column is referenced by its index?

We have a NUMBER column that has a width of 25 in a table and is indexed. The table itself has ~ 43 million rows in it.

We find that referencing the number column in question can take much longer in certain cases, and we suspect its where the length of the number in this column is closer to the maximum length of 25.

Someone working with me mentioned that an index references only part of the field and not all of it, and that it may be based on the first 16 characters / numbers. Obviously if our number exceeds this length then the index will loose efficiency and might even have no use at all.  I've researched this claim myself but have drawn a blank.