End characters getting deleted

[busman84]

I am using the ui-mask for a phone field. If a user fills out the field with the proper digits, lets say (323) 555-1234, is it possible to set the mask to no longer accept any more input unless the user actaully erases a digit. Currently I am experiencing an issue where if the user goes into the middle of that number and types a 7 then the last digit will be erased. Using the previous number as an example it would show up as (323)575-5123. It erased the four from the end of the number and I would like it to not even register the 7 unless the user erases one of the previously entered digits. Thanks for any help.