SMS email: 

There are two options to send SMS to drivers.

1. Twilio messaging subscription service. This is a third-party paid service. See http://twilio.com
   This is faster than the email option and easier to use because you need the cell phone number and there is no need to know the cellular service provider @domain name. You get the credentials from your Twilio account.
   
   
   
   
2. SMS via email. This is free since it's sent using your email account.
   
   You need to enter the the email address of the mobile device in the drivers profile, then click on the Email button on dispatch window and choose Send Trip as a Text Message (SMS) to Driver.

The driver profile has an entry for SMS Email Address.

Here are the email addresses for some of the most common cellular phone carriers in the US:

xxxxxxxxx is the digit phone number, no spaces, no hyphens for example: 2125551212@txt.att.net 

If the carrier is not listed, consult their company website.

SMS or text messaging is a convenient way to send brief text message directly to a mobile phone.
A text message can be up to 160 characters long or about 25 words. If you are sending SMS messages longer than 160, you need to upgrade your SMS service to allow more characters by contacting your wireless phone provider. We recommend you get a data plan with unlimited email and data.

If the SMS email you sent is not received by the mobile phone, contact you wireless service provider. They may have to provision your phone to accept SMS.