At the moment as you pointed out, you can set each one individually, however VoXalot is not yet configured to automatically handle fallback....
{Ring VoXalot, if not available Ring SIP URI, if Not Available Ring Number} is not supported for the time being
However you can manually switch between ringing VoXalot when you are near your ATA, and ringing your Cell when you are away (just before heading out you'd have to de-activate one forwarding rule, and activate the other)
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