I Don't think this is a dsp problems, voip and fax trigger different things so one might work when the other don't.
It looks like your 2851 is trying to send faxes using cisco fax relay and the recieving end might not be configured for that. Also, faxing over ip is a challenging issue so one should try to eliminate it as much as possible.
Not sure what your topology looks like, but try to avoid teho of fax calls over the wan, have a separate route pattern for the fax that sends it via the local gw always.
As for further troubleshuting, the following url might help:
Thanks I saw this docu and trying to troubleshoot this issue but w/ no luck:(
There are a few remote sites in this installation w/ 2851 routers w/ the same HW configuration and all these branch offices fork fine w/ fax machines connected to the VG248. All settings (IOS version, CCM settings, dial-peers) are the same (rechecked many times).
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...