My broken phone (IELTS complaint letter)

"You should spend no more than 20 minutes on this task and write at least 150 words.
You have bought a mobile phone in a tax-free shop just a few days ago, and it doesn’t work properly. Task: Write a letter to the manager to complain about it and ask him to solve the problem.
You do not need to write your address. Begin your letter as follows: Dear Sir/Madam, "

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Dear Sir/Madam,

This is a standard address when you don't know whom you are writing to.

I'm writing to you to express my dissatisfaction with a product, that I've purchased in your store in London airport`s duty free zone.

Last Sunday the 11th of August, I've returned back to Moscow from London. While waiting inside the duty free zone I've decided to buy the MiMiMi Phone7 (awesome name! ha!) in your store. After two days of using my new phone I've realized, that the volume hardware button just had stopped working. I was very disappointed to see this latest expensive model with a 2-years warranty breaking down so soon. I was almost ready to accept this little inconvenience, but unfortunately this fault was just the beginning. Three days later my phone stopped to connecting to wifi networks. You can imagine how these failures upset me.

Here's a very simple explanantion for using present perfect: it's only for talking about the present.  Because you are telling a story in the past, you should use past simple (and past continuous and past perfect). You'll never give a time reference for a present perfect action.

<em>Stopped to connect</em> means the phone finished one action, and then connected. <em>Stopped connecting</em> means the connection is the finished action.

The seller in your store assured me, that although the store itself doesn't provide any warranty to customers, I'd still be able to make use of MiMiMi's global warranty in my home town. Two days ago I've decided to contact the local MiMiMi representatives, but the manager in charge refused to repair the phone and informed me, that this particular model is not eligible for your global warranty terms.

I insist on the urgent repairing of my phone, otherwise I'm going to write a complaint to your supervisor will be obliged to take the matter further.

The last line looks like a blunt threat. We threaten with much more discretion in formal English. :)

Yours sincerely, (I don't know why you should be faithful to people who are messing you around.)
Vladimir

Beware of throwing in those Russian commas. We use commas sparingly in English.  If we can help it, we try to avoid using them at all.

In general, this was pretty good.  Just pay attention to the usual tricky things such as perfect tenses and articles.