And? .............

Why don't you give us some more information.
What is the Field's datatype.
What Format do you want? Remember, this is an international newsgroup
and none of us know what your telephone numbers look like.
What have you tried?
What has been the result?
etc. etc. etc.

Probably would help to know which version of Access you are using to verify
that it contains an input mask for telephone numbers, however, all should let
you customize one.  You also should tell us how you want the phone numbers to
appear.  Do you need to include Country codes?  Do you want to use
parentheses around the area codes?  Do you need formats for internationsal
numbers, etc.