1) There is no upper limit for most jobs about the employees can be "on call" or even work--the exceptions are certain regulated fields where safety is an issue, like truck drivers and pilots, or when there is union contract limiting this. So you can be asked to be on all as much as they like.1a) An employer can make it a condition of employment that you return all calls and fire you if you don't.2) You have to be paid for any hours you actually work. So say you are called in for a repair and work 6 hours--even if you theoretically should be paid for 4, you need to be paid for the 6 you worked.3) IF you are restricted in where you can do and go while on call, it MAY be the case that you have to be paid for at least some of them--if you have limitations, such as you have to be able to get to the hotel within 10 or 15 minutes of a call, you may need to be paid for more hours. If this is the case, speak with an employment attorney to see what rights you may have.4) Whether or not it makes sense to cut you or pay an outside person is a business decision, not a legal issue.