Quitting Google Voice via Google Does Not Remove Google Voice From Your iPhone
Even if you go through the very out of date and confusing steps provided by Google to remove and stop the use of Google Voice, Google Voice is not removed from your iPhone.
Therefore when you receive a call that you are unable to answer the callers will hear a recording informing them that the number is no longer in service.
The "phone number" no longer in service is the Google Voice Number, not Your Number.
Unfortunately, to fix the issue and remove Google Voice from your iPhone you must call AT&T advanced technical support.
CALL AT&T ADVANCED TECHNICAL SUPPORT: 1-800-331-0500.