i doubt it's illegal. as Lostn51 pointed out, they are company phone lines monitored in pursuit of company interests. it's no different than tracking charges on a corporate credit card, mileage and stops in a company car, etc.
we don't get paid to make personal calls, to gossip in the halls, or to do anything else that isn't directly related to the jobs we were hired to do. i'm the first to admit i don't devote every single minute at the office to things that are 100% work related. but if i got "called out" for it, i don't feel i'd have a right to complain.
did you know that many companies also have software installed on their networks that monitor everything you do on the computer? every stop you make on the internet highway; every email you send; every keystroke you type.
is it sneaky and creepy to record you without letting you know? probably. but, is it wrong for them to protect their own interests? no.
look at it this way ... if you aren't doing anything wrong - and i assume you are not - you have nothing to worry about.
so ... if you had something to worry about, would it be their fault?
why be nervous? just keep doing a good job and you'll be fine. :wink: