HTTP Status Code error When Sending SMS in Civicrm-Wordpress
Check the phone number and see if there is the correct country code.  The code was not adding the 1+ for US/Canada yesterday, April 26th.  See comments on the file in GitHub for a temporary workaround:  https://github.com/PalanteJon/org.civicrm.sms.clickatell/commit/ef4e4119b76331ee90bf547044607822907aa323#commitcomment-17262754

Here's a PR request with the workaround:
https://github.com/gah242s/org.civicrm.sms.clickatell/commit/aa74097cf66510710b23c22bae5735db59c156db