Some services such as didww.com have a quirk in their software that only allows SIP URI forwarding to sip addresses that have phone numbers, e.g. firstname.lastname@example.org. I've heard that Voxbone operates the same way.
It would be nice if we could create an alias to our extensions in that format when needed. As it is, I'm doing something like creating an FWD account, SIP forwarding to it, and registering FWD as a trunk. It is just one more hop for a call and one more place for somethig to go wrong.