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: 
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.
In the US, in most if not all states, the Judge at a jury trial may not comment in such a way as to indicate a belief in the truth or falsity of testimony or the guilt or innocence of the accused.

I believe the rule is different in the UK and perhaps elsewhere.

However this was not in the presence of a jury.

**Addition**:
This is because in a jury trial, the jury, not the Judge, is supposed to determine the facts. The drafters of US Procedure apparently thought that any comment by the judge would be highly influential with the jury. I think that this was a reaction against 17th C and 18th C British practice.

In England and Wales at least it used to be the case that the Judge would routinely comment on the evidence, both during the course of the trial, and during the "Summing uP" which followed the evidence but I think came before the arguments of the lawyers. I am not sure if theis is still the procedure.