You are always free to keep any field you want blank. Often times churches will record either the husband or wife's cell number in this field and that becomes the family contact number as well as the member's cell number. 

Note: The only field required in the Family Record is Last Name.

If this is a family that has a student in RE and your church is currently using that module then we suggest adding at least one of their cell numbers into the family Telephone field.  This is the number that shows in the RE Module as the Family Contact.