Same here - I hate talking on the phone and I find it really easy to get distracted or lose track of the conversation. I like text formats because it's all clear and in front of me. This is a preference, though, and some people just as strongly prefer voice communication! The most common reason I've heard (besides the inconvenience of typing) is that you can read emotions better over voice. For example, something that is intended as a joke or sarcasm might not come across correctly in text, but be very obvious when said out loud. This is very important to some people, and they may feel that phone conversations are more personal and effective.
As with all relationships, being sensitive to other's needs and compromising where you can is best!
A few days ago, a friend asked me to explain a technical concept to him via phone. During the conversation, I lost the subject and began mumbling, which I think made him feel that I'm not familiar with the concept and that I lied to him about it; today I was told that he asked someone else about it again (which is kind of rude from my opinion).
This sounds like you and your friend have different communication styles, rather than your friend intentionally being rude. For example, I would rather read a tutorial than watch a video of one, but that doesn't mean I think less of the guy who makes videos. Although it does hurt when a friend goes to someone else, try not to take this one personally.
If it happens again over the phone and you would like to keep discussing with your friend, let your friend know you're aware of the communication problem and ask for another chance:
(mumble mumble)... Oh geez, I know I'm probably not being very clear here. I tend to get distracted on the phone. It's easier for me to talk about this stuff (in person, over email) because that helps me keep track of my thoughts better - do you have some time later / would that work for you?
Now you and your friend can work out a solution.
I occasionally receive calls from strangers who want to order something or negotiate about project, work, school, etc. I don't want them to, because it's just simply difficult for me to talk via phone. However, I don't have any problems at all meeting with people and talking face-to-face to them.
What should I do?
In these situations, why not suggest that? Since they are strangers, they won't be aware of your preferences until you tell them.
Thanks for your interest! I'd be happy to discuss this but phone really isn't the best way to reach me. My email address is firstname.lastname@example.org or if you'd prefer we could set up a meeting?
Depending on their situation, they can suggest which is better and you can go from there.
Another option is to tell the person you are on a time limit when you answer the phone (the limit being however long you can manage talking on the phone). Then when the time is up, thank them for the conversation and ask if you can follow up via email. Especially in workplace settings, people are understanding of time limits and follow-up emails are quite common, so this usually works well when a phone conversation gets too complicated.