Good question!!! We have the same sort of calling card number for long distance calls and users around here are concerned about making these numbers visible. They're supposed to be kept secret. We also want to prevent these numbers from appearing in any log files.
For example, if calling card information is specified as a trailing comma-prefixed number string (perhaps with trailing whitespace) then the following rules would strip it off when creating an displayable representation of the dial string:
Using the dialtest program to test out the rules:
Note that on the server the session logs must be protected to insure the unadulterated dialstrings are not readable by normal users. This is done using the LogFileMode configuration parameter; e.g.
If the faxcron administrative script is run on the fax server machine, then it's also a good idea to shorten the AGELOG parameter that controls how long session logs are kept around. Either that, or the script could be changed to automatically strip privileged information once it's no longer needed for debugging failed calls.