Missing Recommendation Letter I am applying to Grad School. I have indicated three references. However, one of them has not yet completed. He has stopped responding to mails and phone calls too. Would it be wise to change him with some other reference ? 

If it is of any importance, the person in the question is my undergraduate thesis advisor. 

Since I don't have commenting rights I will add some more info here. 

@kostia I visited his office some days back -> he disagreed to write then. Then some days after I visited, he writes to me saying he can do 5. And so I indicated him in requests. That was the last I had heard from him. I write him reminder e-mails almost every day, he doesn't reply.
Adding to @Kostia's answer, a negative letter from the undergraduate thesis advisor will tank your application. 

He is giving all the signs of not being able to write a positive letter. He disagreed when you first asked him, then he agreed to only writing a few, now he is avoiding you.

I would absolutely not use him for any letters if at all possible. Again, a bad letter from your advisor is worse than a missing letter. Try to find a third person and have them write to as many schools as will let you change the letter writers.