Here are a couple of solutions that you can accomplish without getting deep into the system:
1. If you have access to a desktop app like Phone Manager or CallViewer, you can use it to control an extension your calls point to, then have it go off hook and dial all the digits. To forward phantom ext 1234 to 555-555-6789, for example, you'd dial 359 (remote program code), then 1234 (ext), then password, then 355 (fwd all calls), then 8 (outside line), then 5555556789.
2. REALLY old-school, but you could put a modem on one of the built-in SL ports and do the above procedure as DTMF.