There's no obvious reason that this specific behavior would occur. It would make sense at this point, to just factory-reset your device and start over from scratch. It isn't that hard.
First, log into your Google voice account on your laptop/desktop web browser, and go here:
https://www.google.com/voice#phonesConfirm that you see the exact two words "Your number" on the left side of the web page, with your correct Google Voice inbound number. Do you? If so, confirm that you have a check mark next to "Google Chat". Stay on this page, and leave this web browser tab open. Press Ctrl-T to open a new browser tab.
Log into the OBiTALK web portal,
http://www.obitalk.com/obinet/pg/obhdevDelete your OBi 202 completely (click the trash can icon). Wait a few minutes for OBiTALK to remotely reset your device. Do not add it back yet. Pick up the phone attached to the OBi and enter ***0 then 1# to hear it read back your firmware version. Note the last four digits (the build number). If lower than 5285, then download 5285 from this link:
http://fw.obihai.com/OBi202-3-1-0-5285.fw. Log into your OBi from its local IP address (press ***1 to get the address, and the default login is admin/admin). Go to the System Management --> Device Update page, select the file you downloaded, and upload it. After the OBi is finished updating, press ***8 then 1 on the phone to restore it to factory defaults.
Next, add it back to the OBiTALK web portal, by clicking the "Add device" link on the left side of the page, and following the instructions.
Click the link to configure Google Voice on the OBi and follow the directions. When doing so, be absolutely sure that the pop-up confirmation window shows the correct Gmail user ID as is shown on your Google Voice page.
Did that resolve the issue?