This is most likely the result of a regional incompatibility. Even though there have been many efforts to standardize communications across the globe, each region of the world tends to do things just a bit differently to everyone else, which leads to problems.
Where are you located? If for example you imported a voice modem manufactured for the North American telecom system, and then attempt to use the device in Britain for example, the device may fail.
The reason is, many different regions use different tones and ways of signalling such events such as hang-up,busy,Caller ID, etc. This means modems that are not specifically built to be compatible with the system standard used in your area may have trouble detecting some of these events, and therefore it may result in recordings of dial tones and operator instructions as you point out.
To give you a specific example.. as I understand it, the North American Bellcore system leaves the phone line completely dead when all phones are on the hook and it's not in use. In countries like Australia however, I believe certain pulses of tones are sent down the line when it is not in use to let the devices know on the other end it is still 'active' (i.e. physically connected to a phone service) but is simply not being used at that particular moment. When you use equipment built for North America on a line that is sending tones to keep it active, the equipment could easily misinterpret this as actual call activity and therefore fail to hang-up.
I suggest checking with your manufacturer to see if they can provide alternative drivers/firmware for your region.
Edit: Also you should check the End of Call settings in the line's properties window, here is a quote from the IVM manual:
Use the end-of-call tone detection from both the hardware and its internal software detection. The default settings usually work well but if you find that calls hang up unexpectedly (during playback or record) or you have good hardware and want to reduce call online times, tick the Enable end-of-call tone detection during playback option.
Enable end-of-call tone detection during playback By default this is off to stop the out-going audio being mistaken for an end-of-call tone by cheaper voice modem circuits. If you have a professional telephony device and are playing long recordings you might try turning this on so that DialDictate will disconnect immediately if the caller hangs up during playback. Otherwise it waits to the end of playback.
Enable software based end-of-call tone detection By default this is on so DialDictate detects the end-of-call tone even when the hardware fails to detect it. This must be on if end-of-call tone detection during playback is off.