Thanks for using the Apple Support Communities. After reading your post, I understand that you have some concerns related to call forwarding. Specifically, you are unable to turn call forwarding off. I know how important it is to receive calls on your iPhone and would be happy to help!
Call forwarding is a handy feature, and how to enable and disable it will vary depending on your carrier's network type. The following information about call forwarding is provided in the Call forwarding, call waiting, and other call features on iPhone article:
If you're not sure what network type your carrier uses, you can reach out to them directly to find out. They may also be able to assist you in turning the feature off if there is not an option to do so directly on your iPhone.
Cheers!