Yes, you're correct. I got a "promotion" to a higher-level admin job in my organization. The job description made no mention of covering the office phones or covering the phone of the person I directly support. Also, in the interviews no one mentioned answering calls.
About 3 weeks into my job, I had 7 other people's phone lines put on my phone. Every time they get a call, it rings on my phone. If they or their assistants aren't around or step away, I have to answer their lines. At least 20-30 times a day. Very annoying.
Bottom line, even if the job posting doesn't list answering phones, you'll probably have to do it anyway. Of course, some offices have a higher call-volume than others.