When dealing with a disgruntled customer, my main ideology is to make sure to hear them out. Most disgruntled people are disgruntled because they feel like they haven't been heard, and so silence is my first step to allow them to speak their piece. More times than none, they're not angry at me or really trying to vent towards me. They just need to get it out. After that, carefully listen to key things they're saying and make sure to let them know that I understand their frustrations and try my best from the key things that I've heard to repeat back to them solutions that might fix or alleviate or relieve the issue. From there, pretty much normally I'm not having too many issues. After that, customers more times than not just want to be heard, so the main thing is just practice active listening, grace, and patience. If you cannot alleviate or relieve, make sure to just let them know that while you understand their frustrations, protocols, standards, or procedures are this way, and try your best to advise the movement further.