On call compensation

First of all, your employer is not responsible for paying for your cell phone bill, regardless of the fact that you must use it for work (although you may be able to deduct such time on your tax return). Secondly, if an employee must be on-call outside of the workplace, then they may have to be paid pay for that time. If the worker has constraints placed on them so that they have little control of the time that they are on-call (i.e. they cannot use for their own enjoyment and/or benefit), then that employee should be paid. As a general rule, the more restrictions that are on an them, the more likely it is that they should be paid.
 
Some considerations taken into account as to whether not this time that should be paid are:
 

The amount of calls that the worker gets while on call (the more they get the more likely the time must be paid);
The amount of time in which the worker has to respond after receiving a call (the more immediate the response must be, the greater the likelihood that they are to be paid);.
The geographical limitations, if any, as to where the worker can go while on call (if they must stay within a limited distance from the workplace, the more likely is is that they need to be paid for their time);
The restrictions, if any, as to what an employee can/cannot do while on call (the more there are the greater the liklihood that the worker needs to be paid).

 
For further information, you can contact your state's department of labor.