We send text messages all over the world, so to ensure that you're sending your SMS to the right country you must make sure that your contacts are set out in international format. Don't use '00' or '+' at the start, just simply start the number with the international dialing code (44 for the UK, 1 for America, 91 for India etc). For example an English mobile number, 07712345678 would become 447712345678.
Don't worry! If you already have hundreds of contact numbers stored in an Excel document then you don't need to go through each one individually. Have a look at - http://www.textburst.com/blog/formatting-contact-data-for-upload-into-textburst to read about how you can format all of your numbers at once.