If you typically answer the phone or greet customers with ‘ … can I help you?’ – nothing wrong with that. It’s short, courteous and prompts an answer. Good for you. But the answer could be ‘no’, since that’s one of the implied choices in this ‘closed’ question.
I prefer simply adding a ‘how’ – ‘ … how can I help you?’ The inference with this ‘open’ question is ‘of course I can help you … what do you need?’ Just as clear and short, but harder to answer with ‘no’.
Consider this option to enhance first impressions, since customers are influenced most with what they hear first. That’s why I Love ‘How’