In 6.0, under the country settings, you can change this by clicking on the
"send originator's Caller Id".. this is the default. This does not exist in 5.0, the default is that we do send the originators caller id and NOT the transferring party caller id which you are getting.
One thing to check is a file on your NBX located in /dos0/R5_0_31folder.
ftp into the nbx and get that file off. Check and see if perhapsthe "Originating Caller Id = off " is there.. If it is, then change it to ON. If there is not "Originating Caller id", that means it is by default turned on.
Perhaps the telco is modifying the caller id and sending the main caller id of the number.. Requires further investigation.