That's no good for people who only have the voip number, I'm fussy who has my mobile number.
The good providers will soon be letting you redirect your landline (ie VoIP or POTS) number to your mobile anyway. Look at what Microsoft are doing with one of the major carriers in the US where they are using the Microsoft LIVE services to set your presence information & then route calls accordingly.
Voice over IP includes the SIP (Session Initiation Protocol) mechanism for setting up & tearing down calls via IP. SIP enables all of the presence & rerouting stuff so it becomes part of the package once we move to IP across the piece
Sneaky or what?
You've been able to do that for landlines for several decades. At a price.
I'm not sure it's decades, but it's been around a while in the POTS world. The whole VoIP piece drives us away from being known as a number, to being known as an SIP end-point, rather like a domain. As such it's you VoIP will ultimately be trying to contact, not your 01234 567890 number.