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 > Voice Mail. Click Create Voice Mail.
Select the dropdown for Greeting Type, then click Call Me. Your forwarding number (i.e. your cell phone) will ring. When you answer it, you will be prompted to record your greeting.
Voice Mail: This is the standard message that callers will hear during your available times.
Unavailable: Callers will hear this message during hours that you have specified you are unavailable.