When you select Call Using with an external phone number in VOD, the incoming calls directly connects to Voicemail.
Virtual Office Desktop
You may set Call Using to the computer or the desk phone to continue receiving inbound calls.
If you have desk phone, the call will get connected on the desk phone if Call Using is selected with an external number. The call will go to voicemail, if you have soft phone only.
You can not use Call Using set to an external phone number and receive inbound calls. This is by design.