I have heard other people in my 12 step fellowship tell me that my voice does not give a good first impression, and that they had no idea that I did not have an intellectual disability until they got to know me better. It really was that direct a couple of times. I have also heard people describe my voice as being monotone.
That still doesnt answer a particular question though: How do you know it was your VOICE giving them that impression? People... particularly NTs... dont just read your voice. In fact, most arent very good at that part. Facial and body language is the first thing noticed and the bit most heavily processed.
Heck, the fact that you mention "monotone" rather says to me that it ISNT your voice. Lots of people speak in monotone, particularly those in very professional settings. "Monotone" is not a traditional indicator of mental... anything. Instead, it's an indicator of someone who is very "serious". Which would, actually, be beneficial to you, if you must be calling people as part of your job. If someone actually thinks there's a mental disability because of that... then it's not you who has the issue. "voice doesnt give a good first impression" also doesnt count. That, in fact, is likely the bit about lacking confidence that I mentioned earlier. When you hear interviewers say something like that, that's almost always what they mean, for instance.
But also... how do you know about what people OUTSIDE of this fellowship thing think? The fact that it's coming from a specific group, and not random individuals from all over the place, is itself quite suspicious. Note that, particularly among groups, ideas spread even if they dont make sense. If someone thinks that of you... just one person... and they start telling others that (others who previously had no impressions either way) behind your back, or sometimes right in front of you, suddenly those people who were told so will also "hear" it, or "remember" hearing it. It can even happen if the original person doesnt ACTUALLY think that, but simply lies. It's not that they (the others it spreads to) are ACTUALLY hearing it in your voice: it's that their mind has now registered that they're SUPPOSED to hear it. Yes, this is bloody stupid. But people do this all the time. I've always found it to be more than a little irritating to watch groups of otherwise smart people do this.
That's why I mention the impressions of random, non-connected individuals. That effect cannot be present, in that case.
Other than that though... just stop worrying about it. Worry about it too much, and you'll start actually inserting it into your voice. You wont do it on purpose, obviously... but it can happen that way anyway. "Self fulfilling prophecy". Actually, better than that... dont just stop worrying about it. Stop CARING about it. You have a job to do. It doesnt matter what others think, so long as the job gets done. The more you get the job done successfully, and the more you realize how well you're doing at it, the more your confidence will rise. And the less this problem will be present.
Anyway, that's all out of me on this one. I dont have too much time for forum-going currently, so... yeah, that's enough. Good luck to you, sir. I hope it all goes well.