Not to insult your intelligence, but I have to ask...

Let's say your Voipo number is 9195551234 and your assigned password is hg2y08x1.

Your register string should look like this:

9195551234:hg2y08x1@central01.voipwelcome.com/9195551234

You should have an inbound route that directs 9195551234 to a valid extension or ring group. (which it sounds like you already do)

Have you considered setting up a 'catchall' route to your desk phone? (Any DID/Any CID)