The information in this thread is useful, but only to a point because it's incomplete. The syntax suggested does in fact re-route anonymous calls, but it also has the effect of not ringing the attached phone for any other calls and instead sending them to voicemail. This is because the required ,{ph} was not appended to the directive. Further, accomplishing this feat with only Google Voice accounts apparently requires the creation and use of three accounts and not just two: the first as a primary line and phone number for SP1, the second as an account to enable SP2, and the third to provide a forwarding number distinct from both SP1 and SP2. (Note that the account used to create SP2 doesn't necessarily need a Google Voice incoming phone number created, if SP2 is only used for this purpose.) This is in because calling a Google Voice number from its own account or associated forwarding phone(s) automatically drops into voicemail management. In my experimentation I found that it could be done with just two accounts, if the syntax is modified to {?:sp1(sp2number)},{ph}, but only if one doesn't mind having the attached phone ring ONCE with every forward. I didn't try a stunt using just two accounts and the syntax {?:sp2(sp1number)},{ph}, simply because that's just so cyclical it seemed destined to fail, but perhaps even that would work.
So the complete syntax to accomplish this using (three) Google Voice accounts is:
SP1 Service | X_InboundCallRoute = {?:sp2(GV3number)},{ph}
where GV3number is the incoming-calls number created for the third Google Voice account, and that third account would also have the Google do-not-disturb feature enabled to send calls straight to voicemail (for the announcement). This will block anonymous calls to SP1 and ring the attached phone for all others. If this is the only documentation for this technique, at least it's fully documented now.
P.S. The same could also be done for the PSTN LINE, if that landline has Caller ID service active:
LINE Port | InboundCallRoute = {?:sp2(GV3number)},{ph}
GV3number in this case could probably be replaced by sp1number and still work fine, but if you already have that third GV account and number for the foregoing why not use it there also?