xRoachx gave very good advice for an input mask. That will work for new
records.
If it's existing records that are wrong, you need to actually change the
data to see the formatting at the table level. If the data is consistent,
like 1234567890, you could format it to look right in queries, forms, and
reports.
However if you are ever doing business with foreign countries, I suggest
forgetting about the format of a phone number. Aside from parts North
America, most other countries phone numbers do not follow any set pattern.
For example my old phone number in England looked like 0869 233123. You will
have troubles trying to get an input mask to work with all the different
formats.