I have an old Moto G5 android on monthly contract . Call forwarding was enabled then turned off but the phone still diverts incoming calls to the forwarding number last put in. Call forwarding on phone is described as off. Trying to put in a new call forward number ( no number currently listed) returns the message "unexpected response from network". O2 suggested ##002# but it returns error message. Tried sim in different phone, no different. Tried removing battery, no different. Looked on my O2 but can't see device settings in case wifi calling is enabled as suggested by someone. In despair. O2 said someone would ring back but of course they did not.
Ask O2 to disable wifi calling on your account if you're not using it.
You should then be able to use ##002# to cancel all diverts.