Well, I really fixed it now. First I thought "oh, I'll just change the DN of the DID destination to a non-conflicting number, and create a speed call to send DID calls to that number. That doesn't fix the problem, however, because now the speed call conflicts with the DN. What I did was RTFM, and discover that there's a mechanism for sending DID numbers to DNs without the use of speed calls. First, enable the DID service in the Trunk Attributes form (easy to find), and then in the Direct Inward Dialing form, add all the DID numbers (in my case, the PSTN number minus the first 4 digits), along with the internal DN that you want it to go to. That worked in the end.