I was finally successful in remotely setting call forwarding on my LINE port which has AT&T POTS service:
1. From my cell phone (which is in my OBi circle of trust) I dialed my google voice number which is on SP1
2. The OBI attendant answered, I select 2 to make a new call, and entered mypin#
3. For the number I entered: **81172nnnnnnnnnn# where nnnnnnnnnn is the number to forward to
After the # is entered a new call is placed to nnnnnnnnnn. In this case it was my cell phone. I heard call waiting tone and clicked over to answer the call. Then simply hung up. I may be imagining this but it seems like the call placed during the call forwarding process has to be answered for it to take. I cannot test forwarding to an 11 digit number (including a 1) because I don't have a default long distance service on the AT&T line. I sure thought I tried this combination before, but I don't think I ever clicked over to answer the call waiting call on my cell phone that was coming in as part of the call forwarding process.
To remotely cancel call forwarding I performed the same steps except I entered **81173# and got 3 short tones and a dial tone which is the same thing I get when cancelling locally.
More info from AT&T instructions:
Turn On — To activate Call Forwarding, press 72# on your home phone. Dial the number you want to forward calls to. When the call is answered, Call Forwarding will be activated. If no one answers or the line is busy, press 72# again, redial the number and Call Forwarding will be activated.
So it wasn't just my imagination the call has to be answered unless you repeat the process a second time. Frankly I've never personally had a need to use call forwarding. I guess I should have RTFM first, huh? (grin) But, now that I know I can set it remotely from the OBi without having to pay even more to AT&T for the privilege... it could be useful.