i dont think its possible , on some machines you can add prefix , on other not...
maybe you can create an addressbook with the nr 7 already in ?
I would like to have the outside line prefix automatically added when a user inputs a fax number to send a fax. Currently every phone number must have a "7" at the start to get an outside line in our PBX. I looking to setup the system so the users don't have to do this. They just have to enter the number to fax too and the system automatically adds the "7" to the number when it dials. I have seen this feature in other systems, but I don't see where it can be configured in the 7232. Is it possible to do this?