enough bandwidth available
sorry to burst that bubble: but robotic voice is almost always a bandwidth issue. it also generally relates to g729 (which is generally only used in low-bandwidth situations).
but let's have a look at your statement: enough available. I have this sneaking suspicion that you have not "shut off internet" entirely during an episode of this problem. As in: if it isn't Vicidial, it shouldn't be happening. No Twitter, no Youtube, nothing. Set it at the router to disable all internet packets going anywhere other than Vicidial. The next time Mr. Robovoice pops up ... flip that switch and see if Mr. Robovoice disappears suddenly. This includes the owner's iPad (even if his kid is playing a very important Video on it ... LOL).
You'd be surprised how easy it is to change "enough" bandwidth into "too little" for a short period of time during a website's burst mode download or upload.
And if you have not done it yet, try speedtesting the internet connection (I like broadband.gov's consumer broadband test ... it's administered by the FCC to gather data to force providers to increase their bandwidth and actually provide what they claim!). If you try that during the problem, continually checking, you may find that your bandwidth "dips" (Had that happen a few times ... suddenly have 0.1M bandwidth for 30 seconds will wreak havoc on a dialer!)
Happy Hunting