New Here - 1st Post: The mirror Sirius gave Harry in Book 5

[errx2]

No: HPFGUIDX 119599


This may be a stupid question but. . . it has been bugging me since I 
finished the book months ago.  At the end of the Book, Harry finds 
the mirror Sirius gave him and tries to call Sirius.  Could he have 
used the mirror all along to reach Sirius instead of the fire, etc.?  
If not, why not?  If so, why didn't Sirius just remind him to use the 
mirror?

If anyone is kind enough to answer, please be gentle with me.  I read 
all the books one right after the other and have not reread any, 
yet.  I guess I'm a late bloomer, and I'm glad of it too.  I would 
have gone nuts waiting years for the next book.

errx2