Ending calls help please

Hi all,
since iOS12 I believe I am unable to end phone calls. Occasionally in iOS11.4 I had this issue but now I'd say 95% of the time I can't end a call. Two finger double tap does not work. Infact, it seems like the entire screen simply locks as I can't access the keyboard or do anything. I have to turn off VO and get a sighted person to end the call or shut down the phone. I have two phones so I don't believe it is a phone issue. Both iPhone 6 and 8 behave in exactly the same way. Is there any other way of ending calls? or, has anyone else seen this. The power button hasn't ended calls for ages I think so I'm stuck and this is a bad bug if that is what it is. Any help appreciated.