Our latest update on iOS can experience issues with enabling and/or updating. We have been in communication with Apple and this is a known bug with them. Apple offered a workaround that seems to work (and it doesn’t require an uninstall of Mr. Number which causes you to lose your blocklist). So, if you are experiencing issues after updating to the latest version, please try this:
Wait more than one hour after Mr. Number gets into a bad state. Reboot the phone, then try to enable Mr. Number again and it should correct itself.
If this doesn't work then try the fail-safe at the beginning of this article.