Thank you for your question today, I look forward to assisting you. I bring nearly 20 years of legal experience in various disciplines.
The ADA does not require that an employer remove required functions of a job. It requires that an employer give reasonable accommodations that would allow you to still meet the same requirements as everyone else, just in a different way.
For instance, if you had a vision problem the employer doesn't have to allow you to not produce the same amount of computer work as everyone else, but rather, the employer would have to give you a special computer that woud work with your vision problem.
So, the issue is not about reducing the standard, but making it so that you can meet the standard. If your position is that you can't do the required on call due to your disability, that is not a "reasonable accommodation" because it is not about helping you meet the standard, but rather, you are asking that the standard be removed entirely due to your disability.
Generally speaking, the ADA is not going to require that. Now, you can challenge whether or not it should be a required task, but if everyone else has to do it too, it's going to be legal to require you to do it as well. You have not said that they are targeting you for this extra duty, while not making others do it. That really cuts against any sort of ADA discrimination claim.
You can certainly contact the EEOC
to have them review the situation. You can allege that they have refused to grant you a reasonable accommodation. The employer will state that the accommodation is not reasonable, because it forces them to create a position that has a required function removed. The EEOC can agree with the employer or you, but on just the facts you've outlined the EEOC is most likely to find no cause for an ADA claim. I'm sorry.