If you just get call forwarding on your main line, before you leave at night, you pick up the main line, dial 72#, get a stutter dial-tone from the telephone company, and dial the off site number that you want all calls to go to. The call will ring through, wait for answer at the other end to make the call forwarding take effect. If there is no answer, or the line is busy, hang up and do the same sequence again, this time you will get a double beep confirmation tone. For this, you do not need Centrex service.
If you want calls to be answered by your Partner System and Partner Mail's automated attendant first, and then give the caller an option to press something to transfer to the off site location, you WILL need Centrex. You would set a selector code to do a "Centrex Transfer" to the off site number. When the caller presses the selector code, the Partner Mail will "flash", pause, dial the programmed telephone number, flash again, then disconnect, which will complete the transfer.
Another option would be to have an "emergency" mailbox with outcalling to the person on-call.