If a DDI is assigned to a phone then you cannot call it from outside and bypass its ringing without setting call forward all on the DN, you could designate another DDI to reach this DDIs voicemail directly from outside, but I dont see the point of it. If the desire is to send calls directly to someone's voicemail internally the most common practice is to use * prefix followed by the internal extension which matches a CTI RP that strips the * via voicemail profile and forwards to voicemail, but this would only be applicable to internally dialed numbers as you are in control of the dial plan not the telco provider.
