Thanks Jonathan.
We worked it out - basically the users local profile contained our inbound primary customer number. With admin, we'd enabled the users 'basic information' to be shown within contact directory. As this persons profile had our inbound number, it was wrongly assuming via work that any calls into this number were from this user. We had to remove the users profile, remove the incorrect number within the profile then re-add them to ensure the problem was resolved, which it now is. Thanks for your time contributing to the resolution.