Camilo, you were likely speaking with a rude american.  In the United States, people get angry when they call customer service and a foreigner answers.  Many americans believe that if they call customer service that another american should answer the phone.  These people likely never learned a foreign language or had very little contact with others from different countries.  These people despise accents because they have to work harder to understand what is being said.  Americans in general also do not use proper English a lot of the time, use phrases that don't translate easily, word their sentences strangely, and speak very quickly.  They get angry because they cannot communicate in an easily understood way and blame others who don't understand them.  They have a superiority complex.  Americans can be extremely xenophobic, not to mention racist, and be very open and condescending about it.  They are not always welcoming to outsiders.

Also, you were trying to speak to people from a country where only 12% of the population has proficient literacy skills. According to a study conducted in late April, 2013 by the U.S. Department of Education and the National Institute of Literacy, In the United States, 14% of the population can't read. 21% of adults in the U.S. read below a 5th grade level, and 19% of high school graduates can't read. Our literacy results haven't changed in over 10 years.  Americans are not the smartest people to try to talk with.  if 1 in 5 people cannot read, imagine how terrible their language skills must be.

Thanks for your comments!

Speed is not a problem. Accents are more difficult to understand. 

When "sales" sounds like "fails" and "services" sounds like "finances", they may speak as slowly as they want, I still won't understand them.

I have always had a hard time speaking English on the phone. I don't know why but I am not able to understand anything. Although, I used to think that my English listening skills were good because I could understand movie and TV series without subtitles. From my experience, they always get mad at you when you ask them to repeat something and at least those I have had to talk with tend to be rude and yell 'Is there anyone who speaks English at thia company?'.

When I work with people who are learning English or are not native speakers, I try to speak slowly and use basic sentence structure.  I try to keep my sentences in Subject-Verb order and simple.  I start new sentences instead of conjoining them.  This seems to be easier for the listener.  I also avoid vernacular, using only easily translated words.