That's one of my main problems with cell phones in general. We're still (behaviorally) used to in-home phones, where people call less frequently and more importantly, so interrupting your current task to answer it made sense. Now, social calls are frequent, and people still interrupt whatever they're doing to answer the phone. They're artificially high priority.

I let everyone know that phones, with me, are low priority - I have an answering machine. Call logs. I can see who you are and when you called whenever I have time - call me twice if it's actually important.

But I'll still check my phone to see who's calling me when it rings, because doing so is such a minor interruption and might be important and not everyone will try again - I'll probably pick up if a hospital or police department calls me. It's not saying "I have more important people to talk to than you", it's saying "Talking with you is not the most important thing that can possibly exist at this time", which is true and expectable - wouldn't you like to know ASAP if your family is in the hospital?

I don't think it's as rude. If my phone goes off I check because it might genuinely be more important - if I excuse myself from a social conversation because a client website just went offline, I don't think that would be considered rude. (If it was a social message / call, the AFK interaction obviously gets priority)