I have a Nokia 6100 phone. I also have a 3 year-old son. They are clearly designed to work as a package. If I'm checking my voicemail and he is nearby - singing, shouting, crying or laughing - the pitch and timbre of his voice exactly match the tone made by the '7' key, and so the messages get deleted.
If you've left me a message recently and you haven't heard back, that must be why. Sorry.