This functions uses a number of rules to test if a text string can be accepted as a phone number.
##CASE(ISPHONE(Phone),1,Phone,SKIPFIELD())
ISPHONE(Phone)TRUE, otherwise FALSE.CASE(condition, value_if_true, value_if_false)ISPHONE(Phone) condition: Phone contains a valid phone number (ISPHONE(Phone) = TRUE), the formula returns 1 and therefore the field is updated with the phone number. Phone does not contain a valid phone number (ISPHONE(Phone) = FALSE), SKIPFIELD() prevents the field from being updated.