We are logging SMS messages and one particular user shows up in the SMS logs with only the outgoing part of the conversation. The strange thing is the "To" field shows it going to a 6 digit # (not a BB PIN) where it normally shows an actual 10 digit phone number. Every other user logs incoming and outgoing messages with actual phone numbers. This user is part of the same IT policy as everyone else.

My guess was this is from some other 3rd party app on the phone but I checked his device on the BES for installed applications and nothing stands out.

Any ideas on this behavior?

have you checked if an updated firmware exists for this BB ? This sounds like a bug of the OS.