1.  A man answered the phone. He said Mr. Willings was away. WHO  

The man WHO answered the phone said (that) Mr. Willings was away.

2.  I missed the train. I usually catch this train. THAT  

I missed the train THAT I usually catch.

3.  He introduced me to his students. Most of them were from abroad. WHOM

He introduced me to his students, most of WHOM were from abroad. 

4.  The car had very bad brakes. We were in this car. THAT  

The car THAT we were in had very bad brakes. 

5.  Mr. Rossen said he was too busy to speak to me. I had come especially to see him. WHOM  

Mr. Rossen, WHOM I had come especially to see, was too busy to speak to me.

6.  His girlfriend turned out to be a German spy. He trusted her absolutely. WHOM  

His girlfriend, WHOM he trusted absolutely, turned out to be a German spy.