SQL & PL/SQL

HI
I want to get a number of 15 digits from a column where the user only know the last 10 digits.

So that when the user enter a number with 10 digits, only the record of that specific number should be displayed.
And then when the user did not enter any number all the records in the table should be displayed.(this part works fine)

The problem is: when the user enters any last digits( last, second last, thirth last and so on) the records which satisfy this are retrieved, which is not supose to be the case.
I' am trying to use the substr function in the where clause but I'm not sure if it's working or not becasue the result of the query is just the same as before i used the substr.

Thanks