No. Neden is a question in itself.

Q words:
Niçin
Neden
Ne
Nasıl
Nerede
Nereye
Kim
mı (and all other forms with vowel harmony).

e.g. neden geldi? Why did he come?
Nereye gittin? Where did you go to? do not need mi with them to make them a Q.

How about making it into a question:

Ben kimin o şeyi yaptığını neden şimdi anladım?

This could be a line from a detective to his team in a TV show:

Why do I now understand who did that thing?, or more colloquially (How come I now understand who dunnit?

PS I wouldn't have got this close without the clues from taksim and aslı above!

U R smart well done!

What are you doing:

sen : you
ne: what
yapıyorsun: you are doing.

The first "sen" isn't totally necessary in the sentence as we have the "sun" ending on "yapıyor" which shows it is you we are talking about.

"ne yapıyorsun" is enough to say "what are you doing".

Putting "sen ne yapıyorsun" either makes it clearer, or is used to emphasise the you bit, e.g. If they have said what they are doing, they might ask "sen ne yapıyorsun" what are YOU doing.

you are doing

How do you tell if it is landline or cell?

Turkish mobile/cell phone numbers start with 05...

++ 90 5....

what comes next depends on the operator: e.g. Turkcell is 0532, 0533 etc

No problem