Automatic SMS for AfterHoursVoicemail

Hello,  

Please see attached call flow.  I want to be able to send an automatic SMS after hours. I have setup in workflow with a Call Received trigger for Ext 4 to activate after hours, but the trigger is not getting hit and the caller is not receiving an SMS.

Is this because Ext 4 (AH VM) is a Message Only Extension?  If so, what are my workaround options to consider?  

Thanks in advance for your help!

-Kashif