A person in the apts that I resides have entered my apt on at least two occasions without notification while I was at work. No request to fix things or no need for them to enter my apt.

Is it legal in NC for a landlord to enter apt without written notification or a phone call to my cell, which they have, or must they provide said information prior to entering apt? If so how much notice are they suppose to give the tenant?

No they may not unless something in your written lease gives them some right in it. They are suppoe to give notice or have some work order or some emergancy need.

A person in the apts that I resides have entered my apt on at least two occasions without notification while I was at work. No request to fix things or no need for them to enter my apt.

Is it legal in NC for a landlord to enter apt without written notification or a phone call to my cell, which they have, or must they provide said information prior to entering apt? If so how much notice are they suppose to give the tenant?
You must receive notice prior to entry into the apartment. And then you have the right to state whether you need to be in the apartment. However, for entries that include inspection of fire extinguishers etc, they must notify you 24 hours in advance and then you have the option to place the extinguisher outside of the apartment so there is no entry.

Write a formal letter to your leasing manager and cc: the District Manager (if there is one) and send it certified. Therefore, if anything is missing, you have documentation that the property has been notified that illegal entry was made into your unit. For better reference, Landlord Tenant law books/ phamphelets are located in the reference section of your local library. Just ask the librarian.