It's equally annoying and rude when either customers or staff do it.
I do expect the assistant's undivided attention when I am paying or have a query. I had to interrupt an assistant's call to/from her mother to query a price that came up on the till. She was not best pleased. But if she had been paying attention she might have seen that she had scanned an item twice and I wouldn't have had to interrupt her. If I hadn't noticed she would have overcharged me by nearly £5. I worked on the shop floor in both Boots and M&S years ago and we weren't allowed to take phones or money on the shop floor.
Is it rude to not finish a book club choice that was selected by someone else?
When a political leader lies on their CV - can you trust them?
).
