There are a number of 3rd party applications, such as Robot/ALERT from Help Systems that will send SMS text messages from the AS400 (or iSeries, i5, whatever).
An alternative, if you don’t want to (or unable to) spend the money is to use email. Most cell phone companies will accept email addresses addressed to firstname.lastname@example.org and forward them either as emails or SMS messages to the phone.
If the cellphone you want to send to accepts emails this way you can use the SNDDST command to send an email message from your AS400 (iSeries, etc).
To use the SNDDST command you need to configure SMTP on your AS400. I have a document on how to do this at www.astradyne-uk.com/docs/14-02001.pdf which may help.