Actually, my concern was that TXC was going to try to ping my server, or do something similar to prove there really was a PBX at the address I had given them, and would cancel my request when my box ignored them. However, I got my "Welcome Packet" later in the day today, so it turns out my concerns were unwarranted.
That said, I honestly could not get the DIDs to work using the conventional chan_SIP trunking page in the GUI. I believe the reason is that TXC always sends an INVITE request to port *5060* when setting up an incoming call and port 5060 is owned and operated by pjsip on my system (Incredible PBX 13-12.3). To work around this problem, I resorted to creating the TXC trunk under pjsip, something I'd never done before. Pjsip doesn't have a "Peer Details" window like the conventional SIP and the correlations weren't obvious. The important setting is "SIP Server", which should be set to 216.66.23.179 (or whatever address TXC gives you).
This got the DIDs working, but it isn't smooth: Each time Asterisk reloads, pjsip attempts to register with TXC. This attempt is rejected, and semi-permanently blocked if "Permanent Auth Rejection" is checked. The other glitch is with Qualifies: The URI that pjsip cobbles together for the OPTIONS request ('sip
216.66.23.179:5060') is invalid. I bumped the qualify interval up to a large value to minimize the error messages that pjsip was dumping into the logs every minute. But the important thing is that both my DIDs are now working.
If anyone can explain how to clean up the Qualifies under pjsip, or better yet how to convert this trunk to the more conventional Chan_SIP, I'm all ears, but for now it's working well enough to fiddle around with.