You can't. You can, however, infer that 3 or 4 digits are passed based on working DID numbers. For instance, if someone has an extension of 432, and their DID is 555-1432, then it passes 3 digits. If their extension is 1432, it would be passing 4. On a PRI, it is possible to manipulate the digits somewhat, and add or delete digits according to a desired scheme. TipHelp@charter.net