Both your sentences are different in the way they are structured using words, phrases, and clauses. Both of your sentences are correct.
In English grammar, we have the Eight Parts of Speech that are words, and then we put those into phrases, and finally, we use them in clauses. There is much to say about all this. And your sentences can be explained with these ideas in mind.
Now I study about relative clauses. But I have found something wrong.
Again, there's nothing wrong.
In your first sentence you took to two separate sentences and combined them. This is called sentence combining. You took the second sentence and made it an adjective dependent clause: "...who can speak six languages." This sentence structure is called a Complex Sentence. Your two original sentences are called Simple Sentence. It is diagrammed like this:
I | met | a woman | who can speak six languages
Subject | verb | article +direct object | adjective dependent clause that modifies "woman" to say something about the woman. The relative pronoun "who" refers to its antecedent (the word the pronoun refers to), which is "woman" = "....woman can speak six languages."
In the second problem, you took the first sentence and put it into another adjective dependent clause, a short one: who phoned = man phoned.
This too is a complex sentence. I'll give you a link to show you the four kinds or types of sentence we mostly use in the English language. They have to do with clauses; that's what makes them different, the different clauses used.
Main clause: A man did not give his name
Dependent clause: who phoned
Main clause + dependent clause = Complex Sentence
A | man | who phoned | did give | not | his | name.
Here is an excellent page to help you understand what a clause is and how it is used.