Question: I was recently party to a preliminary hearing in a criminal case in which a 911 call was played. The content of the 911 call was very beneficial to the defendant. During the hearing, the judge said something to the effect of, "It would be a question for the jury whether the caller was 'being honest.'"

In other words, the judge was suggesting that the caller to the 911 was being disengenuous and deliberately conveying a false impression -- something that I think would never occur to most people because the call seemed completely genuine. Also, the caller would have had no motive to faking their mental state. I got the idea that the judge was looking for some angle to discredit the 911 call out of interest to see the defendant convicted.

In any case, from the judge's remark I got the idea that he might say something like that during the trial, or give the jury an instruction designed to make them think the 911 caller was putting on an elaborate act.

Is a judge allowed to do this?

Answer: 
It is the judge's obligation to instruct the jury w.r.t. believing witnesses. [This](https://govt.westlaw.com/wcrji/Document/Ief97fa32e10d11daade1ae871d9b2cbe?viewType=FullText&originationContext=documenttoc&transitionType=DocumentItem&contextData=(sc.Default)&bhcp=1) is the introductory instruction for criminal trials in Washington, which on that topic says

> 
> You are the sole judges of the credibility of each witness. You are
> also the sole judges of the value or weight to be given to the
> testimony of each witness. In assessing credibility, you must avoid
> bias, conscious or unconscious, including bias based on religion,
> ethnicity, race, sexual orientation, gender or disability. In
> considering a witness's testimony, you may consider these things: the
> opportunity of the witness to observe or know the things he or she
> testifies about; the ability of the witness to observe accurately; the
> quality of a witness's memory while testifying; the manner of the
> witness while testifying; any personal interest that the witness might
> have in the outcome or the issues; any bias or prejudice that the
> witness may have shown; the reasonableness of the witness's statements
> in the context of all of the other evidence; and any other factors
> that affect your evaluation or belief of a witness or your evaluation
> of his or her testimony.
> 
> 
> 

Some such statement will be made in any trial. There used to be a more specific instruction in witness credibility, but it was withdrawn. The general instruction also says

> 
> Our state constitution prohibits a trial judge from making a comment
> on the evidence. It would be improper for me to express, by words or
> conduct, my personal opinion about the value of testimony or other
> evidence. I have not intentionally done this. If it appeared to you
> that I have indicated my personal opinion in any way, either during
> trial or in giving these instructions, you must disregard this
> entirely.
> 
> 
> 

Except for Texas and West Virgina, all states have such instructions. If a judge went off the rails and said "You are going to have to decide if you believe all that stuff that Smith said", that would be reversible error. The judge may not imply belief or disbelief, and may not make comments that tend to favor the defense vs. the prosecution.
There is a concept of [implicature](https://en.wikipedia.org/wiki/Implicature) that says that meaning is conveyed not only by the meanings of the words, but by the circumstances that are likely to cause someone to utter those words. There is nothing in the literal meanings of the words that says that the witness is lying. Your belief that it conveys that seems to be based on implicature: the judge would not feel compelled to mention it unless they thought the witness is lying.

However, that is not necessarily the case. Certainly if for one witness and one witness only, the judge were to say this without anything else giving them reason to, it could come across as implying that the witness is untrustworthy. On the other hand, if the judge were to say this each time a witness takes the stand, or say it at the very beginning of the trial and indicate that it applies to all witnesses, or say it when prompted by something more than just the witness testifying, such as a party asking the judge to make a decision that they believe relies on an assertion of fact, then this inference is less valid.