Say you have two phone numbers {981456234 and 345234981}. So, now phone number filed becomes multi-attribute. But in a relational model, this can't be stored in a single row. It will be stored as
Shimpy 981456234
Shimpy 345234981
But there can be other models like ER model where data can be represented as
Shimpy - {981456234 and 345234981} which is not in 1NF.