To quote from the user guide

To forward your calls to another station or group
1. With the handset on hook dial #11
2. Press the DSS button of the station that will receive your calls
3. Dial #

Your SPK button will flicker as a reminder that you have forwarded your calls.

To cancel

1. With the handset on hook, dial #11
2. Press your DSS button
3. Dial # again. SPK button will stop flickering