One should not respond to a greeting or repeat what the caller to prayer is saying. He may speak if  there is some necessity (i.e., to guide a blind man who fears he may be harmed).

If he sneezes, he should praise Allah to himself and simply move his lips (without making a sound).

Ibn ‘Umar (R.A.) related that a man passed by the Prophet (PBUH) and greeted him while the Prophet (PBUH) was relieving himself. The Prophet did not return his greeting. (This is related by "the group," except for al-Bukhari.)

Abu Sa’eed (R.A.) reported that he heard the Messenger of Allah (PBUH) say,

"Isn’t it true that Allah detests (to dislike intensely) those who converse while they relieve themselves?"

This was related by Ahmad, Abu Dawud and Ibn Majah.

This Hadith seems to support the position that it is forbidden to talk. Many scholars, however, say that it is only disliked, not forbidden.

Fiqh-Sunnah :  Volume 1  Number 17b

Jazak’Allah Kh’air to Br. Jamal.