How to send text message from desktop to a phone number?

If you are trying to send e-mail to text.  That service is blocked with the exception of verified companies who are confirmed with T-Mobile to not spam.  

If you mean text to text, as mentioned above.  Digits is one of the best ways of doing so.  There are also a few Android SMS/MMS apps, like Pulse, that offer that feature through various methods of subscriptions.  You could also try Google Voice as a free alternative.