Employment Law Questions? Ask an Employment Lawyer.
Thank you for your question.
This is certainly a very serious issue and you are right to be concerned. You need to document this to the upper level management and blow the whistle. The best thing to do is to write a formal letter to the upper level management (your boss's boss) and tell them about what is going on. You will need to let them know each and every time that your boss has done this, stating the dates and times. This should get the ball running on trying to get the situation resolved and also give you some cover the next time your boss makes the allegation that you are not answering the phones.
If push comes to shove, you could sue your boss for fraud if they terminate you, but you need to try to collect some hard evidence that your boss is the one who is turning off the ringer.
Please let me know if you have any further questions on this issue and please remember to rate my answer positively.