So - first caveat - many knowledge work jobs have the "Other duties as required." in there, or they are so commonly implied that some duties may be standard for the industry and hard to argue. Example - test your code when you write it. It's hard to say "I'm not QA" - there's an implication that you will deliver minimally working code and that involves some degree of testing.
And there are some basic duties that are commonly understood by humanity that it's fair on occasion to expect that anyone, in any role, would do these things by virtue of obligation to society. My thought would be - try to use a fire extinguisher to put out a small fire, or pull the fire alarm and help others get out of the building - you don't have to be called a fireman to do these basic things, and it's never part of an engineer's job description, but it IS an expectation.
These cases, boil down to "accept it when":
- It's a basic cultural value needed to glue society together.
- It's a direct connection to do doing things that ARE in your job description, particularly when it comes to doing your work efficiently and at a basic level of quality.
From there the options for task avoidance depend on particulars of the situation.
Infrequent, Short Duration Tasks
But if this comes up so infrequently that the time to do any of the proposed alternatives would take longer, skip it as just not worth it.
Infrequent, Long Duration Tasks
Ask about prioritization. If we're talking about 2 weeks of work that will delay 2 weeks of work that fit your job description - ask why. Often a boss may miss that the work you were actually hired to do is the key work (isn't that why they hired you, after all?), and have lost site in the heat of an emergency. Try to make sure that you sound like part of the team here, and not a prima donna. "Is that critial to our mission? Is it more critical than this other thing?" is better than a scornful "Is THAT the thing I should be working on? Is it really worth my valueable time?" is a worse plan.
Keep in mind, the answer may be yes. If this two week not-your-job task the difference between bankruptcy and solvency for the company, then it's also the difference between you having a job and not having a job - because regardless of whether they fire you or not, you won't have a job if the company has declared bankruptcy. That's the extreme example, but there are plenty of cases that can fall on this side of the "do it anyway" line.
OTOH - if the answer is "yes" and the task is so long that you could find a new job in that time --- start looking. They don't value the work that you value, and it's time to find a place that can meet your career goals.
Frequent, Short Duration Tasks
Keep an account for some time. How much of your day is this taking? How many interruptions come per week? What is the impact on your actual work?
Once you have a sense of the actual impact - you can triage this as either not sufficiently impacting that you can afford to ignore it, or something that detracts meaningfully from your ability to perform the job you were hired for. Talk to the boss about it - does he understand and agree with the frequency of this task and it's impact on your work? Do you have any ideas for changing it? The work is likely necessary, but maybe there is another way to do it that works better, is faster or is done by someone else. The "find someone else to do it" option is always there, and very tempting, but you may get more mileage out of ways that don't increase the team headcount.
Keep in mind that bosses can give vague guidance - "do it some, but less..." may be the response. Follow up with a request for specifics - how often? And what happens if the need arises and doesn't get met. Some things (like washing the windows) can be ignored for some time before they get out of control. Other things (like answer the hotline) need a backup plan (get an answering machine?). Yet others (vaccum the floor) can be automated (get a roomba?). There's no one single answer.
Bear in mind that if the task would typically be done by someone more expensive than you are, the answer may be - do it anyway. It's your choice - get good at it, and demand the higher salary (now, or from a future employer), or find a job where you can do things you like doing better.
Frequent, Long Duration Tasks
It's time for a Serious Talk. Your boss has a very different vision for your career than you do. It's time to talk about this. What's the real answer for why you aren't doing the job you were hired for? What's the need in the near term and long term for you to do this other work? Should you change your role, or should you go back to your regular work?
This is a closed door, don't leave without acceptable answers moment.
You may end up leaving. If every day, for many days, and major work, you are not doing the work you were hired for, then either the company needs to own up to it, or you need to look for a new job. The only exception would be if you like the work, the pay or the team enough to compromise on it. But I'm betting that if you were willing to ask the question in a public forum, this is not the case.
A big thing in any conversation is to be aware enough of the specifics to have a meaningful conversation. I was just reading about how different cultures will make decisions in different ways, but in your impression, there is a discrepancy between your expectation and the company's. Have enough information to be able to justify your point. And if the information points out that the impact of the not-job-related work is minimal, give up the battle as there is always some lack of clarity in the workplace.