in the past you could do it using skills. Look like you will soon be able to just do it without skills. however, you can only send to android phones...
Amazon announced today it’s launching a new messaging feature for Alexa devices in the U.S. that will allow you to send texts – yes, SMS messages – to your friends and other contacts using your voice. Customers can now ask Alexa to send a message to a specific contact, and Alexa will figure out how to route it appropriately – using either the previously launched Alexa messaging system, or by sending it out as an SMS instead, if the recipient doesn’t have an Alexa device of their own.
https://techcrunch.com/2018/01/30/you-can-now-use-alexa-to-send-sms-messages-to-your-friends/