This can mean:
1) I'm sorry for forgetting to call you as I promised to do.
2) I'm sorry that you are upset because I called too late after forgetting to call earlier.
3) I'm sorry that you are angry due to PMS of which I am not the cause but due to which I am often in trouble.
Boy: Whoa, she's real SOSsy.
Guy: That kid in The Exorcist is like, mega SOSsy.