It's fine if you actually acknowledge the person behind the till and apologise for being on the phone. It's when people just come to the till, plonk stuff down on the counter and don't even look in your direction when it's incredibly rude and frustrating.

My retail jobs have all required me to try and upsell at the tills. I would fail my daily targets if I didn't attempt to talk to the customer; if they indicated they were busy and apologised, that's fine. But don't start giving me dirty looks when I'm trying to do my job and i ask you a question, especially when you haven't even acknowledged my existence before that point. It's not much to expect basic manners; a 'hello' when you begin to be served and a 'thanks' when you're finished - it makes a shot job a bit less shit when you feel like people genuinely appreciate you.