Although not a news and not really VoIP is still worth mentioning again the way of sending SMS messages from E-Mail clients. The following are the email addresses format for the top six US cell providers:
Virgin Mobile firstname.lastname@example.org
Just replace the phonenumber with the actual number and send the email.
As you can probably noted you need to know the carrier of the phone number. If you don’t know it and you are fine with a small advertisement appended to your SMS you should consider teleflip.com Just send the SMS to email@example.com and they’ll handle the rest.
Alternatively you can use the service provided by sendu.info and send the email to firstname.lastname@example.org
Although these Email to SMS services have nothing to do with VoIP, you can easily use them as a notification services for VoIP applications. For instance if your Email provider allows to forward each received email to a second address you can make use of the Email to SMS feature to receive notifications on your cell each time when a voice mail is delivered to your email inbox.
Comprehensive list of email to SMS
T-Mobile Send SMS online form