It depends whether your standby time is considered "controlled" or "uncontrolled." If you're required to remain on call on the employer's premises or so close to it that you can't use the time effectively for your own purposes, you would be considered on "controlled" standby and entitled to payment for all such hours.
If you're not required to remain on or close to the employer's premises but are merely required to leave word at your home or with the employer about where you can be reached, it would not be considered "controlled" time and you would not be entitled to compensation.
It's a fact-specific question that could best be answered by an attorney who is provided more specific facts. Please contact me if you would like to discuss the matter further.
The answer depends upon the specific facts. The legal issue is how much control your employer exercises over you during the time that you are on-call or on standby. If the level of control is to a point where you are essentially prohibited from engaging in other activities then you probably should be paid. again, the specific facts would be important.