A letter of complaint-any mistakes?

Could you have a look at the following letter of complaint and correct mistakes?

A letter of complaint:

Dear Sir or Madam,

I am writing to express my dissatisfaction with the mobile phone I bought at your shop in London on 28 March, 2012. It was a present from my grandmother on my seventeenth birthday. I chose the model and the colour.
The phone worked perfectly well for the first two days. I could call, answer calls, send messages and even listen to music. But on the third day something happened. When the battery ran down the phone turned off and I could not turn it on.
I was trying to recharge the battery several times but it did not help. So I decided to borrow another battery from my friend. we replaced it but again nothing changed. As the phone is still under guarantee I decided to write to you.
I am afraid I must ask you to check my phone. If you cannot fix it in one week I expect a full refund.
I look forward to hearing from you.
Yours faithfully,