In MyRepChat, if you have the Voice Add-On feature enabled, you can set up a voicemail greeting for when someone calls your MRC number and you aren't available.
From the Dashboard, go to Settings--Add-Ons and Billing, under the Voice Add-On you'll see 'Voicemail'. Click on that, then click on +Create Voicemail:
Make your selection in the dropdown for Greeting Type, then click on 'Call Me'.
Your Forwarding number (i.e. your cell phone) will ring. When you answer it, you will be prompted to record your greeting. You can record a greeting for when you are unavailable, or that will greet a caller when you are simply unable to answer your phone (VoiceMail).