is you page using spry for the phone number validation?
to check this, select the phone number field in design view, if it is using spry, you will notice a blue bounding box. Click that to trigger the spry settings in the property inspector.
in the spry settings, set it to use a custom pattern, and enter:
000-000-0000
set the option to enforce the pattern.
you could also enter a hint it you like.