Changing the length for Exchange 2010 voice mail transcription

Sometimes I get voice mail messages for which Exchange 2010 refuses to provide a voice mail preview. It says “oh, bother; the message was too long.” Wouldn’t it be great to set the threshold for “too long” in Voice Mail Preview?
Too bad; you can’t.
Well, OK. Let me elaborate: you can change the maximum length used when you use a transcription partner. (Wait, what? You didn’t know that you can outsource transcription? Yep. I’ll write more about that later.)
However, there’s no supported way to change the length threshold for messages transcribed on your own system. There’s a good reason for this, though.
All speech recognition systems rely on statistical models to help them “understand” what the speaker’s saying. These models help the speech engine predict what sounds are likely to come next given what’s being said right now, and improved models are a big part of why speaker-independent continuous speech recognition has finally reached the point of actually being useful.
There’s a problem with this approach, though: messages that don’t match the model are really, really hard to recognize. When a speaker says “My phone number is four”, if the next phrase is “syzygy exploding potato” it’s going to be hard for the model to keep up since it’s expecting a few more numbers, not random crapola. The more the message diverges from the model’s expectation, the worse accuracy gets.
This ties into long messages how? Glad you asked. The longer a message is, the more likely it is to be a specialized message– in other words, one that doesn’t match the model well. Consider the difference between a short message (say, “Hi, honey, it’s me. Don’t forget to pick up some milk on the way home.”) and a speaker reading this blog post to your Exchange UM server!
To avoid this situation, Exchange won’t transcribe any message longer than about 75 seconds. That’s not a big deal, given that something like 99% of all voice mail messages are shorter than 75 seconds, but it’s still a bit of a hassle if you need transcription for longer messages. Of course, then you could always use a transcription service, about which more later.