Nanban?

During my last Japanese lesson, my teacher taught me a new sentence "Denwa bango wa nanban desuka"
I understand that it means "what is your telephone number" with denwa being telephone and bango being number.
But what is the meaning of nanban? My Japanese teacher couldn't explain it well and I am thinking if someone here can help me.
Thanks.

In case of phone number, "nanban" literally means "what number(s)".

Then is it possible to say ''Denwa bango wa desuka''
Or is that wrong?

No, that would be wrong.

"nanban" consists of "nan-" (a variant of "nani," meaning "what"), and "ban," referring to numbers. So "nan-ban" is asking "what-number"?

So you COULD say:
- Denwa bango wa NAN desu ka (WHAT is your phone number?)

...but you have to keep the "nan" (what) part in the sentence.

No, that doesn't really work. For the formal/polite phrasing you are aiming for you can't omit it. 何 (nan, nani) is the rough equivalent of "what". "ban" is the counter for numbers, ie "1st" is "ichiban", 2nd niban, etc.

On a side note, not that it comes up much but I more often hear just "... nan desu ka" instead of "-nanban desu ka". Dunno if it is my poor listening skills or what though haha.

If you leave out the word "nanban" then there is no question word in the sentence- kind of like saying in English "Is your phone number?"- it doesn't make sense, does it.