1. answer the phone and ask caller to hold.
2. Everyone in line can see you're the only one available. This will make them angry and some will take it out on you - don't take it personally (unless it is your doing that you're the only one available-if so, consider changing your staffing)
3. Handle each customer in line with a smile
4. Ask the next person in line if you may handle the phone call - if the call will take a while, ask the caller for a name and number and call them back. Or transfer if available.
5. The patron who is being loud has as much right to be loud as the quiet has to be quiet. Tell the customer you will look into the situation and handle it as best you can. A loud patron who is asked to be quiet will usually become louder - instead, offer them a piece of taffy - :=)