Answer: It all depends on whether you have learned sol-fa with the "Fixed Do" method or the "Moveable Do" method. In the United States and England at least, movable Do is the most popular. In Movable Do solfege, the tonic note of a major scale is always "Do," regardless of the key. So in the key of A the note A would Do, B would be Re, C# would be Mi, etc.

But in Fixed Do, as used in France and some other countries (including, I now know from your response letter, Iran!), Do is just another name for C. Re is always D. In the Fixed Do system, the first note of the A scale would be La, because A is La regardless of the key.

In both cases, though, the clef would have nothing to do with it. The notes would be named regardless of clef.

If you think in terms of Movable Do, which is really the most useful kind of solfege, the notes of the A major scale would be A, B, C#, D, E, F#, G#, and they would be named Do, Re, Mi, Fa, So (or Sol), La, Ti, just like every other major scale, which is what is so nice about Movable Do.

But if you were using the Fixed Do method, the notes A, B, C#, D, E, F#, G# would be named La, Si, Do#, Re, Mi, Fa#, Sol#. Aside from being more singable the fixed Do syllables don't really offer any advantage over the letter names.