1) Unplug your OBi. Call that GV number directly. Leave a message, timing it. It should be up to 3 minutes.
2) Plug your OBi back in. After it's done booting, call that GV number directly. Leave a message, timing it. It should be up to 3 minutes.
If both of those work fine, then call you landline number and let it go through the forwarding setup you have (landline to GV to GV). If, when you go to leave a message you get cut off at 21 seconds, it's something to do with your unusual setup.