Is this a reasonable assumption?

that If a person can speak English, that they should be able to understand English? I mean, as long as there are no ridiculous slang words being used, just straight forward English.
For example, if I were fluid in speaking Spanish, wouldn't it be reasonable to assume that I would understand what someone is saying to me if they are also speaking Spanish? The reason I ask is because I get many calls at work by people all around the world that can speak English but often times they do not understand what I am saying. I speak clearly, and plainly, and professionally when people call. I am trying to figure out if I am doing something wrong here.

Whether the person is calling from a country where English is the/an official language or from a country in which he/she has learned English as a second language, you have to remember something... Different accents, phrases, and how quickly a person speaks affect how well another person understands him/her.

Many that have learned English as a second or third language may have learned from textbooks or video but have not had enough experience in conversational English. Understanding vocabulary and some sentence structure does not mean that understanding a native speaker is an easy task.

Before I moved to Per煤 I thought I had a pretty good grip on Spanish from years in a classroom -- but it really did not prepare me well at all. Even after having spent a good time here (and considering myself fairly fluent), there are occassions in which I have to pay special attention when someone is speaking in order to understand them completely.

I do not believe you are doing anything wrong. However, my suggestion to you would be to speak slowly and enunciate words when you come across this problem. Just like it takes patience from your end, the person on the other end of the phone has to have patience as well.

Good luck!

While this is reasoanable, this is not always truth, speaking and listening are different skills that do not necessariy go together for people that have learned Engish as a foreign language.

I speak fairly good English, but sometimes it's hard to understand in certain situations. The lower quality of sound on the telephone may impair comprehension for those that aren't used to it.

At any rate, it really can't be helped. Slow down your speech a little and hope for the best.

No. You made it very clear. I understood every fluid word.