1) If you are on a contract, then this should probably be stipulated in the contract. I'm not contracted, so I don't have set hours or anything so this is no issue for me. But as a matter of principle, I believe both parties should stick to a contract and amend it if necessary. This protects everyone.
2) Depends who has access to the information and how it is presented. As a manager of an IT sub-department, my home phone is published company wide with the explicit warning that you may only call me if you don't get a response to either of two different pagers in a timely manner.
3) Get caller ID and know how to use it. If I don't recognize a number, I always let it go to voice mail. I'll check voice mail and if I deem it important, I'll respond, otherwise I'll just follow up in the morning and say "I was out last night did you ever get your issue resolved?"
Otherwise, I wouldn't make a big stink about it. If you have a receptive boss, it can't hurt to hint that a little extra pay would make you happier about the situation, but I wouldn't rock the boat too hard unless it becomes a problem with people calling you 24x7.