Make sure your phone vendor has a secure pin setup in your PBX, the strongest possible with more than 4 digits would be great. Also consider blocking international service through your carrier if you do not need it. Below is an example of what can happen.
Someone gained access and had our calls being forwarded to a number of other places, and then farmed out that access so that in the span of 12 hours 385 people could use our phone line to call Liberia for a total of 6,800 minutes. So that’s not good. We had been working with the phone service to secure our line, and that has been a struggle. The net result currently is that the Chicago office may not be able to call any toll numbers.