Asterisk with the OpenVox A400P
The OpenVox A400P02 I'd ordered arrived during the week while we were away.
I installed it last night and had a brief play, but wasn't able to get things working with our Vonage ATA. Tonight I made a bit more of a concerted effort, and got everything going with both the line coming out of the Vonage ATA, and the SBC PSTN line. In the end everything just worked, but for some strange reason I had to swap the ports I had the SBC PSTN line and the Vonage ATA plugged into. Very weird.
Driver and configuration-wise, the A400P02 walked and talked exactly like a Digium Wildcard TDM02B, only much cheaper.
All that remains is some serious tweaking of dial plans, to punt the appropriate calls out the appropriate links, and setting up voicemail properly, and I think I can call this project sufficiently completed to move on to MythTV... At a later date I'll get an FXS module so I plug in one of our existing cordless phones.