1. Validate rule that is in Flow action can be used in that a check whether the value starts with either 7,8 or 9 can be stated. Apart from these numbers if it starts with any other number or character error message can be thrown.
2. You can specify property level validation also to check start of the string so that as soon as data is entered and validation is fired and data is verified and error is thrown if the data is not aligned with the stated conditions.