Complex English sentences?

I'm learning English and I was having some difficulty in understanding the meaning of complex sentences in English. I quote a paragraph as follows:
"A new study out from Consumer Intelligence Research Partners estimates that between 300, 000 and 400, 000 total customers who would have otherwise left wound up staying with T-Mobile in the wake of the iPhone's launch on that carrier. According to The Wall Street Journal, CIRP's estimate is based on a survey of 500 phone activations occurring between April and June, with about 75 of those phone activations being on T-Mobile."

I do not understand the phrase "left would have otherwise wound up staying with T-Mobile in the wake of the iPhone's launch on Furniture carrier" is talking about. This structure is very complicated for me. I hope to get an explanation of the native English speakers. I sincerely thank you.

.customers who would have left = customers who were thinking about leaving,
… But, instead,
… These customers would up (ended up) staying with T-Mobile.

So 300, 000 to 400, 000 customers were considering leaving T-Mobile, but because of (in the wake of) the launch of the iPhone at T-Mobile, they decided to stay with T-Mobile.

That sentence would make more sense if you include the previous parts, so that the sentence becomes:
"between 300, 000 and 400, 000 total customers who would have otherwise left wound up staying with T-Mobile in the wake of the iPhone's launch on that carrier."

Now what the sentence is saying is that between 300, 000 and 400, 000 customers would of left if it was not for iPhone's launch on that carrier. One way to gain a better understanding of this question would be to break it into pieces with comma's.

"between 300, 000 and 400, 000 total customers, who would have otherwise left, wound up staying with T-Mobile in the wake of the iPhone's launch on that carrier."

The confusing part of this sentence is probably the middle piece, "who would have otherwise left". When this phrase is placed placed at the end of a sentence, it refers to an event (The IPhone launch) preceding it:

"In the wake of Iphones launch on that carrier, between 300, 000 and 400, 000 total customers, who would have otherwise left, wound up staying."

When it is placed in the middle (So that there's no even preceding it), It will refer to an even further down the sentence, as is the case in the original sentence. One way of looking at this phrase is that it alters the order of the sentence, and is often used to make sentences more "interesting" and create diversity in the passage.

Sorry about my organization, i'm not very good at explaining things, but hopefully this might help you out.

Your problem is that you have attached "left" to the wrong part of the sentence. Let me lay it out more clearly for you:

A new study / out from Consumer Intelligence Research Partners / estimates that between 300, 000 and 400, 000 total customers / who would have otherwise left / wound up staying with T-Mobile / in the wake of the iPhone's launch on that carrier.

It would be slightly better English to say "who otherwise would have left"…

To 'wind up' doing something means 'to end up' doing something. "They intended to do A, but instead they wound up doing B."