To clarify thing the 07744 number is only useful if you are dialing international numbers using your mobile phone.
You will dial one of 07744 numbers and then the voice will ask you to key in your destination number. Its like a calling card without the pin code etc. Best things is that from Orange and T-Mobile you can call these numbers using your free package minutes.
If you are on O2 you can call one of 0870 numbers and you will get international calls from there.
Hope this clears things about 07744. Also read the first forum at top which have basic information about these numbers.
For cheapest international sms from O2 I suggest you read
http://www.o2.co.uk/international/callingabroadfromtheuk/pricespaymonthlyHow much does it cost to send a text?
To send a text to an overseas mobile it will cost :
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For customer of ITS and Discount country: 12p (inc. VAT)
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For standard tariff customers: 16p (inc. VAT)