I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. I have been a licensed attorney for more than 3 decades. Please don't confuse excellent service with a satisfactory outcome based on your wants and desires. I can only explain the law to you---not guarantee that you will be happy with the outcome---any more than a cancer surgeon can guarantee a cure.
I think what you are not perhaps getting is that as a substitute you may only get a call once in a while---when they need a substitute. I imagine most of the regular drivers are very hungry for work and you may only rarely may get a call now that you are a sub. I can't explain why they won't return your call, but I suspect it is simply because they will call you when they need you and they don't want to be bothered by your calls asking for work.
If you had been fired, they would have told you I expect. You are not just a sub driver and you will only get a call if they need you as a substitute. I see nothing unusual about this, and there are no facts suggesting that you are being discriminated against, or retaliated against. You simply moved from being a regular drive to being a sub---which they may have very little need for most of the time.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
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I wish you and yours the best in 2016,