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: 
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.
Of course he can, but in some jurisdictions he will have to choose his words carefully.

Depending on tone or pace of voice, "it's for the jury to decide (this or that)" will often be heard by the jury to mean "but I don't believe it".

Since the meaning depends on tone or pace of voice, who could ever prove anything against the judge?