Google Voice Number Displayed on Phone?
It appears this is a bug with the S3 phone.apk. The XDA thread doesn't have a solution, but there are some things to try...I just don't know if they'll actually fix the issue. You could try installing a replacement dialer. If you're rooted, you could try finding an alternative phone.apk (perhaps from the S2). Finally, a custom ROM built on AOSP (like Cyanogenmod) may not have this issue.