Please beware of sites on the web that offer you the ability to send free SMS text messages via the web.
There are some legitimate ones, but as my dad used to say, there's no such thing as a free counter lunch.
I have found that a lot of the "Free SMS" sites that require you to sign up first, giving your contact details and mobile number are really scammers and do not actually let you send SMS messages after you have signed up.
Their only reason for being is to trick you into giving your mobile number and/or contact details so that your details can be used to spam you.
Conversely, the sites that don't require you to sign up are less likely to be scammers as they aren't gathering enough info to sell. (short of the number that you have sent your text to).