What you're seeing is caused by one of two issues, and I'm not sure which. You'll have to play around with this stuff a bit.
SMDR is nothing more than serial data being dumped out of the PBX. And every PBX seems to have it's own format for this data. Some of this formatting is controlled by the PBX itself, and some of it is controlled by what fields you software is strapped to look for.
I'm fairly certain that you can pick up the DIALED NUMBER field on those inbound calls. That information--as I recall--shows up in field adjacent to the one that spews out the 771.
So the first thing I want you to do is to play with the SMDR report format. Perhaps you could even post what Call Accounting software you're using. Sometimes you can define the fields themselves, but for the most part, you'll have choices like "AT&T System 25" or "Lucent/Avaya Merlin MAGIX".
The second thing you can do involves a minor tweak to your phone system. Go to Sys Program > Start > Options > SMDR > Format and choose ISDN SMDR. Basic SMDR is enabled by default, and doesn't give you quite the detail that this other choice gives. However, I should mention that the ISDN SMDR option may not have any affect unless you are using PRI on your inbound calls.
Anyway, I'm trying to help you throw mud at this thing. I'm sorry that it took so long to reply to your post.