At any place that takes calls, one must deal with angry callers.  Here are some tips for doing that. 

Listen

The first one is to focus on listening.  Practice active listening: do not talk over the caller and especially do not attempt to argue back.  Give small verbal and nonverbal feedback to show signs of listening.  It is important to let them vent, so keep notes.  Take a few deep breaths while they talk to calm down if needed.  Once they have spoken show you were listening by summarizing and asking any appropriate questions.  Be patient, ask for clarification if necessary.  

Do Not Take It Personally

Remain calm and remember not to take things personally.  Do not respond to angry comments; remember that they are not angry at you, but the situation.  Reassure the caller that their message is getting across and you are being attentive.  By summarizing and focusing on the facts they have presented, you can try to address the issue early before it escalates.  After the caller has vented show that you care and will do everything you can to try and help them.  Focus on an even tone, and try to remain neutral, do not be reactive.  Show empathy for their situation by imagining yourself in their position. 

Avoid Putting The Caller On Hold

Avoid putting them on hold if possible; this can infuriate them further when they speculate about the reason.  Try to keep talking to them about steps currently being taken to resolve the problem instead.  Focus on finding a solution and redirecting the conversation back to it and its main points.  Be honest about what you know and can do; you may not know presently but reassure them you can find out immediately.  Some solutions might not make a caller entirely happy, but they can be asked about a middle ground that will satisfy them if they are kept cooperative.

Remain Positive

Overall, you are trying to make them happy.  Try to remain positive for a resolution.  Give them options to empower them and ask if they have any other issues that might need help when their main problem is addressed.  Add some personality to build a relationship with the caller with statements such as "Let's work together to solve this" or "Let's make sure you get what you need from this situation."