Your post is ambiguous.
Is your POTS line plugged into the LINE jack on an OBi 110, or is this just a regular POTS line with a different telephone plugged directly into the POTS jack in the wall?
If you mean that you have an entirely separate POTS line, then you must have set it up as a forwarding phone number in Google Voice settings.
Log into your Google Voice account from a laptop or desktop computer's web browser, and go to this page:
https://www.google.com/voice#phonesRemove the check mark to the left of the POTS number. If you want to keep using this number to access your GV voicemail, you can leave it listed on that page with no check mark. If you have no intention of using it at all, then delete the number.