As someone pointed out the "call google, enter the number you wish to call, and have google call you back" method of placing a call should hopefully still work. If that is the case then a call-centric/pay by the minute number is all you will need. To make dial your GV number less painful, program it as one of the speed dials. I haven't looked in the ObiHai's manual or done this yet (will when I get home) but you should be able to set a speed dial by picking up the phone and hitting:
*74 following by 1+your google voice number + 2-9 (to set which speed dial it'll be). Then to call your GV number just hit 2-9 (whichever you chose) followed by the # key.
To check a speed dial number you can hit *75 + 2-9.
So theoretically to place a call all you'd have to do is his 2# + dial the number you want, hang up, have google call you back and it'll only cost you a minute (a penny basically) no matter how long the phone call is (I assume <1 min calls are rounded up, if not it'll be even cheaper).