I need your help.
What are we supposed to say when we answer the phone (in a company) or when we work in a shop:
Hello, CAN I help you?
Hello, HOW CAN I help you?
Can we use both of them interchangeably?
Which one is the most frequently used?
Not a teacher.
You can use both, but I think that the second one (How can I help you?) sounds more courteous. I use both when talking on the phone, but I mostly use the first one because it is faster.