So, given my earlier musings, I went ahead and signed up for the $10 a month Engin account (man it's hard to suppress adding an "e" on the end every time I type "Engin").
They finally approved the account today (took about a week or so I think), and I have a Brisbane telephone number. I've also bought the O'Reilly Asterisk book (yes, I've also downloaded the PDF for free, but I feel I should support the author, and I like reading books in dead-tree format).
My next challenge is to obtain a couple of entry-level SIP handsets (recommendations welcome) and to actually get Asterisk up and running, talking to Engin. The pièce de résistance will be to get a TDM card with a couple of FXO modules to plug the standard POTS line and the output of the Vonage ATA (unfortunately Vonage doesn't appear to play well with Asterisk, so I'll have to do this crazy conversion) and have everything terminating in Asterisk.
Of course this is up there with the MythTV box I never got around to building back home, but at least Asterisk is packaged for Debian, so all I have to do is get the handsets and configure Asterisk appropriately. Oh, and maybe actually stay at home for a couple of weekends.