Just building on TomMills' response,the '3' option is not required. From the Specific Tab of the Menu action add an 'F' capability. This will then allow the Menu action to listen for a Fax tone and follow the 'F' path of the Menu. If the F path is then connected to a transfer action routing the call to the fax server, this can be taken care of automatically.
Regarding the number range assuming the extn range is a 4 digit one that does not start with a 0, 1 or 2 (assuming these are used as already suggested) then add ???? to the specific tab of Menu, then attach the exit from this to a transfer action with a target of '$Key' the Voicemail will attempt a transfer to the number entered by the caller. I believe V4.2 allows the value of $Key to be tested against for validity as an extn which would allow the test be made before attempting a transfer.
mmm too much waffle I think, but hopefully of use.