Thank all for the great discussion. I tried a few things after reading your posts.. What I found is similar to what you two discussed.

test 1
1. have grandcentral to initiate a call.
2. "internet call" is automatically started with an incoming phone call.
3. DTMF dialing doesn't work.

test 2
1. start "internet call" manually
2. Dial a phone number using DTMF dialing
3. DTMF dialing works

test 3
1. start gizmo
2. have grandcentral to initiate a call
3. Gizmo rings with an incoming call
4. Gizmo doesn't even provide a keypad for people to press a number during phone call.

Only test #2 works. However, this is not a free call..