If the candidate can't communicate while writing code, and can't communicate about code already written, at some point you have to be afraid the candidate just can't communicate.
If this is a person who can communicate in any situation except an interview situation, then it's a Catch 22 where their ability to communicate in a work environment just can't be verified.
(Unless you secretly record them communicating in some other work context? Probably not a scalable approach.)
At that point, the candidate should not be hired.
If the candidate can't communicate while writing code, and can't communicate about code already written, at some point you have to be afraid the candidate just can't communicate.
If this is a person who can communicate in any situation except an interview situation, then it's a Catch 22 where their ability to communicate in a work environment just can't be verified.
(Unless you secretly record them communicating in some other work context? Probably not a scalable approach.)