Call forwarding is done either globally (on all trunks), or for one trunk only. The default is global – as you found out.
To change to one trunk only, first check which Star Code Profile your Phone Port is using. In the Star Code Profile slot 7 you should see:
*72, Cfwd All, coll($Cfan), set($Cfa,1)
To set forwarding to say SP2 only, change the code in slot 7 to this:
*72, Cfwd SP2, coll(SP2($cfan)),set(SP2($cfa),1)
Now *72 only sets the forwarded number on sp2. *73 disables it.
The only other method is to go into the settings for each spX individually and manually set up:
CallForwardUnconditionalEnable: Check
CallForwardUnconditionalNumber: spX(12345678901)
Note that the number needs to be input in "Trunk Format", define which spX to use for the forwarded call followed by the number in parenthesis.
Using the first method (*72 etc), the forwarded call will always use your Primary Line. The second method you define which spX to use for the forwarded call.
You could allocate a different Star Code Profile to each Phone Port to give yourself some added flexibility.