It depends what the button used to do. I can think of three possibilities:
[ul]
[li]Line Redirection (from the system)[/li]
[li]Call Forward (from the system)[/li]
[li]Call Forward (from the CO)[/li]
[/ul]
In the first case, Line Redirection, you would program a button with the Line Redirection feature (Feature 84). Once it is done, you would press the button, press an outgoing line button, dial the number where to redirect the calls and finally press the line button to be redirected.
In the second case, Call Forward, you would program a button with the Call Forward feature (Feature 4). Once it is done, you would press the button, dial the destination code where to forward the calls and finally press the OK softkey.
In the third case, Call Forward from the CO, you would program an external autodial button with a code (for example, *72) that turns on the feature at the CO. You would also include a pause (Feature 78) and the number where to forward the call.