Phone rejects all incoming calls, how do I fix this?

When your phone rejects incoming calls it is worth checking or making:

-Incoming call settings:
Phone settings->Applications and notifications (or similar option, depending on your phone model)->Telephone/Calls' is the Block all incoming calls option turned off?

-Call forwarding settings:
Phone settings->Call forwarding->are call forwarding disabled or set to the correct number?

-Settings for numbers in the phonebook:
Are call forwarding or blocking set for selected numbers? If it is voicemail forwarding is it active?

-Disable do not disturb.

-Disable aeroplane mode.

- Perform a reboot of the phone.

-Try to update the phone's software.

-Try using a SIM card in another phone, if the problem persists then it could be a SIM card problem or operator restrictions/services.

-Contact your operator and report the problem.