is one of the most frequently used Japanese.
“Sumimasen” usually means “I am sorry.” when you apologise casually but “Sumimaseeen” with the last syllable stretched means more like “Hello! Someone there for me?” or “Excuse me, please.”.
You could say this phrase when you require an attention from somebody when you walk in a shop but there is nobody there, when you stop someone to press shutter for you, or ask directions, or some situation like that.
The proper way to say is “Su-mi-ma-sen” but usually the first “m” is abbreviated. If you stretch “sen” like “seeeen”, it can be used when you draw attention from somebody a bit far from you or somebody does not notice that you are there as well.
You never stretch “sen” when you want to apologise someone as it would sound improper.
Enjoy speaking Japanese!!